Uses back and home button for screen pinning when recents is invisible

All the text and screen pinning hint shows tells the user that the back
and home button should be held to exit screen pinning. The hint also do
not have the recents button shown if it is invisible. The toast code has
been moved to recents and services calls through to post a toast message
depending if the recents button is visible.

Test: manual
Fixes: 72059911
Change-Id: I93abf5072b97760f33e7e77421544a4b3ad27beb