Null-check mNavigationBar when returning its view.

Without this check there's no way to avoid a NullPointerException if
getNavigationBarView() is called before the navigation bar fragment is
created.

Bug: 36852229
Bug: 37330416
Bug: 37331698
Fixes: 36508501
Test: make -j; cannot reproduce bugs.
Change-Id: Ia3c9b4eaea26b78cdc39a446918047eb1f440d70