Fix NPE in NavigationBarView during bugreport

Commit "Merge Navigation Bar and Pie Controls button config (1/2)"
changed the tag field on the navigation bar buttons. This was not
reflected in NavigationBarView#dump() method, which lead to a NPE.

Also take into account that back and recents buttons may not be
present in the navigation bar.

Change-Id: Ie590608a4ad27753b673320132af7ec42d7a2ade