libbinder: RPC flake mode

Add mode to make flakes more visible, for debugging.

Fixes: 190828148
Test: run w/ binderLibTest and the fuzzer for a bit (no tests
  fail - some tests can be made to fail by increasing the delay,
  but this is because some of the tests, namely OnewayStressTest,
  relies on all operations completing within a certain timeframe)
Change-Id: I3b38ff55ffc56410db585fa02e57ac0271da2b5d