Fix bluetooth tests with R8 full mode

Mocking methods with checked exceptions requires an explicit keep rule
with R8 full mode. Add the minimal set of such keep rules, and relax
the temporary opt-out from R8 full mode (that was only needed for these
tests).

Bug: 289285719
Bug: 215530220
Test: atest BluetoothInstrumentationTests
Flag: EXEMPT bugfix
Change-Id: I4782306d0c9e525ed9b329abdeabadce2b51bef2