EdgeBackGestureHandler cleanup

This CL removes some code around EBGH that's not used anymore:
1. the setInsets function in NavigationEdgeBackPlugin
2. the PluginListener logic in EBGH.
3. the navbar exclude region

Bug: 382774299
Test: presubmit
Flag: EXEMPT code cleanup
Change-Id: I46569ffe5576610e16b3174700abff59bbb27e9a