Allow blocking calls to InstrumentationWatcher

InstrumentationWatchers are used in tests, therefore
we are not concerned about them misbehaving on
production devices.

Fixes: 147705910
Test: run an instrumentation test and
  there are no ONEWAY_FLAG warnings from BinderProxy

Change-Id: I323d4598a4d8e5e754251d8d4f27c5c6a2faea2f