Introduce AlwaysUsePrebuiltSdks

Instead of UnbundledBuild, use AlwaysUsePrebuiltSdks
to determine if java modules needs to be built against prebuilt sdks.
And rename UnbundledBuildUsePrebuiltSdks to AlwaysUsePrebuiltSdks to
express its behavior more correctly.(It can be orthgonal to "Unbundled")

Bug: 160390776
Test: TARGET_BUILD_UNBUNDLED_IMAGE=true m vendorimage

Change-Id: I0be7265c1959d8774c295372cd7a9250169f6df9
diff --git a/java/aar.go b/java/aar.go
index ad9b5e7..203de9b 100644
--- a/java/aar.go
+++ b/java/aar.go
@@ -625,7 +625,7 @@
 }
 
 func (a *AARImport) DepsMutator(ctx android.BottomUpMutatorContext) {
-	if !ctx.Config().UnbundledBuildUsePrebuiltSdks() {
+	if !ctx.Config().AlwaysUsePrebuiltSdks() {
 		sdkDep := decodeSdkDep(ctx, sdkContext(a))
 		if sdkDep.useModule && sdkDep.frameworkResModule != "" {
 			ctx.AddVariationDependencies(nil, frameworkResTag, sdkDep.frameworkResModule)