Add onBackStackChanged() checking.

Bug 37433435

We were reporting onBackStackChanged() twice, once when the
operations are collected and once when the operations are
executed. This CL tests that onBackStackChanged() is called
only once for add and pop transactions.

Framework Change: I699c314c40331f4859ea7ff725c5835e07036f18
Test: ran fragment tests

Change-Id: Ife6a3eb12124a5409ca6a1bbc9332c056fe4f30a
2 files changed