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

cc/library.go

@@ -284,7 +284,7 @@ type libraryInterface interface {
 func (library *libraryDecorator) getLibName(ctx ModuleContext) string {
 	name := library.libName
 	if name == "" {
-		name = ctx.ModuleName()
+		name = ctx.baseModuleName()
 	}
 
 	if ctx.Host() && Bool(library.Properties.Unique_host_soname) {