Don't log spurious standby bucket changes during startup

Power-save allowlist is not cached early enough in AppStandbyController
This results in many system apps ping-ponging between EXEMPTED and
NEVER buckets, causing too much logging at boot.

Wait for LOCKED_BOOT_COMPLETED before responding to callbacks from
AppStandbyController about bucket changes.

Also, don't treat NEVER bucket as background restricted. It already
addresses job and alarm quotas. This was another source of bucket
changes due to most exempted apps also being in the NEVER bucket at
start.

Bug: 334353399
Test: statsd_testdrive 441
Change-Id: I2b30970b0467e32f2e8049e72c981b64e129ac1c