Improve threading model for test stability.

There were cases where some async getSharedPreferences() calls would
race with test tear-down, so this ensures we've processed all async
events before executing a test case.

Bug: 190181745
Test: atest BluetoothInstrumentationTests --rerun-until-failure 100
Change-Id: I746afc33e925659cd15ba810337d23d7b359a310