More view hierarchy, fragment debugging.

Add a View.toString() method.

Rename all of the View private flags to have a PFLAG prefix to
avoid going insane trying to figure out which constant goes with
which flag.

Activity.dump() now includes a summary of the activity's view
hierarchy, using the View.toString() method.

All exceptions thrown by FragmentManager now perform a dump of
the owning activity state, where appropriate.

Change-Id: I6482e397e10cb5a0612ab02ce6ed5131823437a6