Fixed some pip state acitvity accounting.

- Corrected ActivityRecord.checkEnterPictureInPictureState() to return
false if the activity doesn't support Pip.
- Wait for previous activity that supports pip to pause before resuming
the next activity with FLAG_RESUME_WHILE_PAUSING so that the activity
has a chance to enter Pip before we resume the next activity. Currently
disabled until we figure-out if callers are passing in the right value
for prev.
- Changed stopActivityLocked() to call setVisible() instead of
setVisibility()...yeah this isn't confusing...

Change-Id: If2336ece4fe5b7104c9b301a4ded5cadc2c2f7cb
Bug: 37370508
Test: manual