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