Fix a race condition that causes BackAnimation to be null.

The race happens between attaching NavigationBarView and initializing
NavigationEdgeBackPlugin. When NavBarView is attached after the plugin
is initialized, the plugin doesn't receive the mBackAnimation instance
passed from NavBarView.

This caused the mBackAnimation to be null and broke back gesture on
tablets.

Bug: 220709601
Test: m -j. Test gesture back on tablets.
Change-Id: I8cf70a24ba5340d27bbd48a8e809d8f922992689