Fix the unbundled mainline module build

This change fixes two problems:

1) the prebuilt apexes are force disabled for the unbundled builds
because we need to build the modules from the source then

2) the dependencies from an sdk_library module to
*.stubs.{public|system|tests} are not added for the unbundled build
because the stubs modules are disabled.

Bug: 137282010
Test: unbundled mainline builds are successful
Test: build com.android.media and inspect the jar file to see if
hiddenapi flags are there
$ cd out/dist/mainline_modules_arm
$ unzip com.android.media.apex apex_payload.img
$ mkdir -p mnt
$ sudo mount -o ro,loop apex_payload.img mnt
$ dexdump2 mnt/javalib/updatable-media.jar | grep hiddenapi
shows results

Change-Id: I2c00af07aac4a15770d3acab011a36e2e4803bfc

java/hiddenapi_singleton.go

@@ -97,7 +97,7 @@ func stubFlagsRule(ctx android.SingletonContext) {
 	// Add the android.test.base to the set of stubs only if the android.test.base module is on
 	// the boot jars list as the runtime will only enforce hiddenapi access against modules on
 	// that list.
-	if inList("android.test.base", ctx.Config().BootJars()) {
+	if inList("android.test.base", ctx.Config().BootJars()) && !ctx.Config().UnbundledBuildUsePrebuiltSdks() {
 		publicStubModules = append(publicStubModules, "android.test.base.stubs")
 	}