Support robolectric prebuilts in android_robolectric_test

The version of robolectric in external/robolectric-shadows is based
on 4.1 and is difficult to update.  Support using the prebuilt versions
in prebuilts/misc/common/robolectric, with the caveat that they won't
support the in-progress SDK version.

Bug: 181886851
Test: manual
Change-Id: Ie3a861cf6d637f97cf00212c02322f92c1043f0a
diff --git a/java/robolectric.go b/java/robolectric.go
index 98bb710..00f233e 100644
--- a/java/robolectric.go
+++ b/java/robolectric.go
@@ -31,13 +31,15 @@
 }
 
 var robolectricDefaultLibs = []string{
-	"Robolectric_all-target",
 	"mockito-robolectric-prebuilt",
 	"truth-prebuilt",
 	// TODO(ccross): this is not needed at link time
 	"junitxml",
 }
 
+const robolectricCurrentLib = "Robolectric_all-target"
+const robolectricPrebuiltLibPattern = "platform-robolectric-%s-prebuilt"
+
 var (
 	roboCoverageLibsTag = dependencyTag{name: "roboCoverageLibs"}
 	roboRuntimesTag     = dependencyTag{name: "roboRuntimes"}
@@ -57,6 +59,10 @@
 		// Number of shards to use when running the tests.
 		Shards *int64
 	}
+
+	// The version number of a robolectric prebuilt to use from prebuilts/misc/common/robolectric
+	// instead of the one built from source in external/robolectric-shadows.
+	Robolectric_prebuilt_version *string
 }
 
 type robolectricTest struct {
@@ -94,6 +100,12 @@
 		ctx.PropertyErrorf("instrumentation_for", "missing required instrumented module")
 	}
 
+	if v := String(r.robolectricProperties.Robolectric_prebuilt_version); v != "" {
+		ctx.AddVariationDependencies(nil, libTag, fmt.Sprintf(robolectricPrebuiltLibPattern, v))
+	} else {
+		ctx.AddVariationDependencies(nil, libTag, robolectricCurrentLib)
+	}
+
 	ctx.AddVariationDependencies(nil, libTag, robolectricDefaultLibs...)
 
 	ctx.AddVariationDependencies(nil, roboCoverageLibsTag, r.robolectricProperties.Coverage_libs...)
@@ -298,7 +310,11 @@
 	if t := r.robolectricProperties.Test_options.Timeout; t != nil {
 		fmt.Fprintln(w, "LOCAL_ROBOTEST_TIMEOUT :=", *t)
 	}
-	fmt.Fprintln(w, "-include external/robolectric-shadows/run_robotests.mk")
+	if v := String(r.robolectricProperties.Robolectric_prebuilt_version); v != "" {
+		fmt.Fprintf(w, "-include prebuilts/misc/common/robolectric/%s/run_robotests.mk\n", v)
+	} else {
+		fmt.Fprintln(w, "-include external/robolectric-shadows/run_robotests.mk")
+	}
 }
 
 // An android_robolectric_test module compiles tests against the Robolectric framework that can run on the local host