Add method to determine variations from a Target

The arch variants are hardcoded in every module type.  Refactor
them out into a Target.Variations() method in preparation for
splitting the arch mutator into two, which will require using
different variations.

Test: m checkbuild
Change-Id: I28ef7cd5168095ac888fe77f04e27f9ad81978c0
diff --git a/android/androidmk.go b/android/androidmk.go
index 9071347..b66fd18 100644
--- a/android/androidmk.go
+++ b/android/androidmk.go
@@ -199,19 +199,19 @@
 	switch amod.Os().Class {
 	case Host:
 		// Make cannot identify LOCAL_MODULE_HOST_ARCH:= common.
-		if archStr != "common" {
+		if amod.Arch().ArchType != Common {
 			a.SetString("LOCAL_MODULE_HOST_ARCH", archStr)
 		}
 		host = true
 	case HostCross:
 		// Make cannot identify LOCAL_MODULE_HOST_CROSS_ARCH:= common.
-		if archStr != "common" {
+		if amod.Arch().ArchType != Common {
 			a.SetString("LOCAL_MODULE_HOST_CROSS_ARCH", archStr)
 		}
 		host = true
 	case Device:
 		// Make cannot identify LOCAL_MODULE_TARGET_ARCH:= common.
-		if archStr != "common" {
+		if amod.Arch().ArchType != Common {
 			if amod.Target().NativeBridge {
 				hostArchStr := amod.Target().NativeBridgeHostArchName
 				if hostArchStr != "" {
diff --git a/android/arch.go b/android/arch.go
index 348b064..5887744 100644
--- a/android/arch.go
+++ b/android/arch.go
@@ -22,9 +22,12 @@
 	"strconv"
 	"strings"
 
+	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 )
 
+const COMMON_VARIANT = "common"
+
 var (
 	archTypeList []ArchType
 
@@ -36,7 +39,7 @@
 	X86_64 = newArch("x86_64", "lib64")
 
 	Common = ArchType{
-		Name: "common",
+		Name: COMMON_VARIANT,
 	}
 )
 
@@ -702,11 +705,23 @@
 }
 
 func (target Target) String() string {
-	variant := ""
+	return target.ArchVariation()
+}
+
+func (target Target) ArchVariation() string {
+	var variation string
 	if target.NativeBridge {
-		variant = "native_bridge_"
+		variation = "native_bridge_"
 	}
-	return target.Os.String() + "_" + variant + target.Arch.String()
+	variation += target.Arch.String()
+
+	return target.Os.String() + "_" + variation
+}
+
+func (target Target) Variations() []blueprint.Variation {
+	return []blueprint.Variation{
+		{Mutator: "arch", Variation: target.ArchVariation()},
+	}
 }
 
 // archMutator splits a module into a variant for each Target requested by the module.  Target selection
diff --git a/android/config.go b/android/config.go
index 26c4e6e..57737f1 100644
--- a/android/config.go
+++ b/android/config.go
@@ -89,9 +89,10 @@
 	ConfigFileName           string
 	ProductVariablesFileName string
 
-	Targets              map[OsType][]Target
-	BuildOsVariant       string
-	BuildOsCommonVariant string
+	Targets             map[OsType][]Target
+	BuildOSTarget       Target // the Target for tools run on the build machine
+	BuildOSCommonTarget Target // the Target for common (java) tools run on the build machine
+	AndroidCommonTarget Target // the Target for common modules for the Android device
 
 	// multilibConflicts for an ArchType is true if there is earlier configured device architecture with the same
 	// multilib value.
@@ -289,8 +290,9 @@
 		config.Targets[BuildOs] = config.Targets[BuildOs][:1]
 	}
 
-	config.BuildOsVariant = config.Targets[BuildOs][0].String()
-	config.BuildOsCommonVariant = getCommonTargets(config.Targets[BuildOs])[0].String()
+	config.BuildOSTarget = config.Targets[BuildOs][0]
+	config.BuildOSCommonTarget = getCommonTargets(config.Targets[BuildOs])[0]
+	config.AndroidCommonTarget = getCommonTargets(config.Targets[Android])[0]
 	config.TestProductVariables.DeviceArch = proptools.StringPtr("arm64")
 	config.TestProductVariables.DeviceArchVariant = proptools.StringPtr("armv8-a")
 	config.TestProductVariables.DeviceSecondaryArch = proptools.StringPtr("arm")
@@ -374,8 +376,11 @@
 	}
 
 	config.Targets = targets
-	config.BuildOsVariant = targets[BuildOs][0].String()
-	config.BuildOsCommonVariant = getCommonTargets(targets[BuildOs])[0].String()
+	config.BuildOSTarget = config.Targets[BuildOs][0]
+	config.BuildOSCommonTarget = getCommonTargets(config.Targets[BuildOs])[0]
+	if len(config.Targets[Android]) > 0 {
+		config.AndroidCommonTarget = getCommonTargets(config.Targets[Android])[0]
+	}
 
 	if err := config.fromEnv(); err != nil {
 		return Config{}, err
diff --git a/android/proto.go b/android/proto.go
index c8ade45..b712258 100644
--- a/android/proto.go
+++ b/android/proto.go
@@ -52,9 +52,8 @@
 	}
 
 	if plugin := String(p.Proto.Plugin); plugin != "" {
-		ctx.AddFarVariationDependencies([]blueprint.Variation{
-			{Mutator: "arch", Variation: ctx.Config().BuildOsVariant},
-		}, ProtoPluginDepTag, "protoc-gen-"+plugin)
+		ctx.AddFarVariationDependencies(ctx.Config().BuildOSTarget.Variations(),
+			ProtoPluginDepTag, "protoc-gen-"+plugin)
 	}
 }