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

apex/apex.go

@@ -1433,10 +1433,13 @@ func (p *Prebuilt) installable() bool {
 }
 
 func (p *Prebuilt) DepsMutator(ctx android.BottomUpMutatorContext) {
-	// If the device is configured to use flattened APEX, don't set
-	// p.properties.Source so that the prebuilt module (which is
-	// a non-flattened APEX) is not used.
-	forceDisable := ctx.Config().FlattenApex() && !ctx.Config().UnbundledBuild()
+	// If the device is configured to use flattened APEX, force disable the prebuilt because
+	// the prebuilt is a non-flattened one.
+	forceDisable := ctx.Config().FlattenApex()
+
+	// Force disable the prebuilts when we are doing unbundled build. We do unbundled build
+	// to build the prebuilts themselves.
+	forceDisable = forceDisable || !ctx.Config().UnbundledBuild()
 
 	// b/137216042 don't use prebuilts when address sanitizer is on
 	forceDisable = forceDisable || android.InList("address", ctx.Config().SanitizeDevice()) ||