Disable all-assigned check in hiddenapi on master-art

Master-art configurations do not have frameworks/base and therefore do
not have hidden API flags. Pass --no-force-assign-all to `hiddenapi`
when frameworks/base does not exist to disable the corresponding
assertion. This enables us to enforce the assertion on non-master-art
builds and also get rid of logspam about missing flags on ART buildbots.

Test: art/tools/buildbot-build.sh on master-art
Bug: 123143676
Change-Id: I074d9554fb11dab3eef904016375730520107ec2

apex/apex.go

@@ -118,7 +118,7 @@ func init() {
 	// projects, and hence cannot built 'aapt2'. Use the SDK prebuilt instead.
 	hostBinToolVariableWithPrebuilt := func(name, prebuiltDir, tool string) {
 		pctx.VariableFunc(name, func(ctx android.PackageVarContext) string {
-			if !android.ExistentPathForSource(ctx, "frameworks/base").Valid() {
+			if !ctx.Config().FrameworksBaseDirExists(ctx) {
 				return filepath.Join(prebuiltDir, runtime.GOOS, "bin", tool)
 			} else {
 				return pctx.HostBinToolPath(ctx, tool).String()