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: Ife6a3eb12124a5409ca6a1bbc9332c056fe4f30a
Change-Id: I699c314c40331f4859ea7ff725c5835e07036f18
1 file changed