Better handle exceptions from BG threads

- Enable TOLERATE_UNHANDLED_EXCEPTIONS by default
- When we detect an exception from worker threads, we prevent subsequent
  messages from getting executed, until the end of the current test.

- Add RavenwoodUtils.waitForLooperDone() and waitForMainLooperDone(),
  which will wait for a looper to be idle, and then throws the pending
  exception, if any.

Fix: 424511030
Flag: TEST_ONLY
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh

Change-Id: I421e0715f888d2cdd5eac2380da40416cde2cefe