feat(HCT): Perform custom migration logic for existing HCT users

This logic is triggered by two scenarios:
(A) During first bootup after OTA update to Android 16, if the
    user had HCT enabled.
  - Trigger: ACTION_PRE_BOOT_COMPLETED.
  - Migration: HCT is disabled and notification is shown.
(B) Restore backup from Android 15 (or earlier), if the backup
    had HCT enabled and new device does not.
  - Trigger: SettingsProvider's restore process.
  - Migration: HCT is not restored and notification is shown.

We store whether the user has seen this notification in a new secure
setting ACCESSIBILITY_HCT_SHOW_PROMPT. This setting is also backed up.

Bug: 369906140
Flag: com.android.graphics.hwui.flags.high_contrast_text_small_text_rect
Test: atest SettingsRoboTests:com.android.settings.accessibility.HighContrastTextMigrationReceiverTest
Test: flash an incremental update on a build with HCT enabled;
      observe HCT is disabled and a notification is sent.
Test: flash an incremental update on a build with HCT disabled;
      observe no change to HCT and no notification.

Change-Id: I4d294ffc0b2eabc59ee7988a579d678975a16380