DO NOT MERGE Remove duplicated onBackStackChanged() call.

Bug 37433435

We were reporting onBackStackChanged() twice, once when the
operations are collected and once when the operations are
executed. This CL only reports the onBackStackChanged() after
the operations are executed.

Test: ran fragment tests
Change-Id: Idaf14cf8c64f87ba78ecd4f0a72a045a30ff9222
(cherry picked from commit ff3deb363de36fc9bab1ec6eb4f0f48e4c3556f4)
3 files changed