Trust agents can show a message to the user on grant

When trust has been granted, trust agents can show
a message to a user when trust is granted with
FLAG_GRANT_TRUST_DISPLAY_MESSAGE.

Previously, the message was only used for debugging.

Test: atest SystemUITests
Fixes: 213911325
Change-Id: Ia449fd98eb0ddac3ea6391c78e76d9e93df8530d
8 files changed