Add visibility for packages/modules/Bluetooth/android/app

The code residing in packages/modules/Bluetooth/android/app is
migrated from packages/apps/Bluetooth and requires the same
visibility rules for frameworks source code.

This path currently does not exist, but will be required for
migration.  Soong build rules do not enforce path existence, which
allows us to add the new path now and not during migration.

Added visibility along with migrated code in a local tree and
performed a local build (bramble-userdebug)

BUG: 196026708
Test: TH
Change-Id: I66cac592489fc1689d45a7b6427bc58c40a824e2