Handle single-choice ChooserActivities and bound ChooserTargetServices

Fix cases where we could try to unbind from a ChooserTargetService
that is not connected. This could happen if we still had stale replies
coming back after the activity was destroyed.

Always offer users an explicit choice in ChooserActivity, don't
auto-start a single option.

Make sure we don't allow a wedged ChooserTargetService to hold a hard
reference to the ChooserActivity via its internal result callback.

Bug 23152483

Change-Id: I7c8b1fc9559dcd477702ef582011b088b07d646b