Create a new product variable to gate blueprint files

Users can use this feature by
1. Setting PRODUCT_INCLUDE_TAGS += <val> in their product mk files
2. Set
```
blueprint_packge_includes {
  match_al: ["<val>"],
}

other_module_type {name: foo}
other_module_type {name: bar}
```

bar and foo will be included if and only if <val> is set

Test: Unit tests in blueprint
Test: TH

Change-Id: I32eed4e3b5ac47fb565c62d13d8881fa984c86f4
diff --git a/android/config.go b/android/config.go
index 4a64b5b..9d9ab30 100644
--- a/android/config.go
+++ b/android/config.go
@@ -1111,6 +1111,10 @@
 	return append([]string(nil), c.productVariables.NamespacesToExport...)
 }
 
+func (c *config) IncludeTags() []string {
+	return c.productVariables.IncludeTags
+}
+
 func (c *config) HostStaticBinaries() bool {
 	return Bool(c.productVariables.HostStaticBinaries)
 }
diff --git a/android/register.go b/android/register.go
index 6c69cc5..6cfc205 100644
--- a/android/register.go
+++ b/android/register.go
@@ -161,6 +161,7 @@
 func NewContext(config Config) *Context {
 	ctx := &Context{blueprint.NewContext(), config}
 	ctx.SetSrcDir(absSrcDir)
+	ctx.AddIncludeTags(config.IncludeTags()...)
 	return ctx
 }
 
diff --git a/android/variable.go b/android/variable.go
index 28f22c9..9725895 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -452,6 +452,8 @@
 	GenerateAidlNdkPlatformBackend bool `json:",omitempty"`
 
 	IgnorePrefer32OnDevice bool `json:",omitempty"`
+
+	IncludeTags []string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {