Initial prebuilt support

Support prebuilt shared libraries as an initial proof-of-concept of
prebuilts.  Future changes will support binaries and static libraries,
and the ability to select which to use based on something besides
blueprint properties.

Test: TestPrebuilts run during m -j
Change-Id: I6e84da667e9005ae11844bad01d25cbe4ced1ce3

android/mutator.go

@@ -33,6 +33,7 @@ func registerMutators() {
 	}
 
 	ctx.TopDown("load_hooks", loadHookMutator).Parallel()
+	ctx.BottomUp("prebuilts", prebuiltMutator).Parallel()
 	ctx.BottomUp("defaults_deps", defaultsDepsMutator).Parallel()
 	ctx.TopDown("defaults", defaultsMutator).Parallel()
 
@@ -45,6 +46,9 @@ func registerMutators() {
 
 	ctx.BottomUp("deps", depsMutator).Parallel()
 
+	ctx.BottomUp("prebuilt_replace", PrebuiltReplaceMutator).Parallel()
+	ctx.TopDown("prebuilt_disable", PrebuiltDisableMutator).Parallel()
+
 	register(postDeps)
 }