Explicitly set Bluetooth to R8 compat mode

This mode matches existing behavior, and is necessary to avoid issues
with mock-related testing of the app target. We will revisit this
after unblocking the global rollout of R8 full mode.

Bug: 215530220
Bug: 289285719
Test: atest BluetoothInstrumentationTests
Change-Id: I9031f80ce223a525a0e25037e9d175ce2cb13f38