Add ability to set onShowListener to custom dialog pref

Currently there is no way to ensure something happens when the
dialog first shows since the getter for it is best effort. This CL
makes it so that a preference can provide an onShowListener so we
can do things right when the dialog becomes available.

Test: robotests in sister CL
Bug: 73102003
Change-Id: I269c3d89dc2323f5d0163cc12208eb8df5ace6f6