Support custom messages for mainline update

Fixes: 293476665
Test: atest BouncerMessageRepositoryTest
Test: verified manually,
  1. Enable feature flag
  2. Reboot the device
  3. Before unlocking, run the command: `adb shell setprop sys.boot.reason.last reboot,mainline_update`
  4. Relevant message should be shown on bouncer.
Change-Id: I3f7029f586013159eff0756798f81048de372098