Trying to come up with a simpler approach for onShow/onHide.

-> In this approach, the shown/hidden state should be tracked
   by the CustomContent provider and excess calls to onShow and onHide
   should be debounced. That is, if onShow is called twice in a row,
   without an intermediate onHide, the second should be discarded.
   Viceversa for onHide.

Change-Id: I2fa48d15f28767775a9a9cb84c1d920867a12176