Show specific messages for why bouncer is shown before or after OTA update.

Test: 1. Restart the device,
      2. Run `adb setprop sys.boot.reason.last reboot,mainline_update` before unlock
      3. See message on bouncer
Test: atest KeyguardViewMediatorTest
Bug: 285956229
Bug: 285578578
Change-Id: I8623cce9e9eb4c321f377f262e9831e3c3a62e93