Backup POWER_BUTTON_LONG_PRESS

Starting with Android 12, start backing up this setting to restore the
user's pref across devices. For users upgrading to a 2021+ device,
they will get the new default (assistant on long-press). If this user
then restores to an older device, they will then get this new behavior
on that device as well, even though it isn't the device default.

Fixes: 189874533
Test: atest SettingsBackupTest
Change-Id: I4b9864045bf12f30da03e3715f5e368bdf48a198