Count modifications to user's power allowlist

Log a counter everytime an app is put into or taken out of the power
allowlist. In normal conditions, this is only expected to happen when
the user toggles this in Settings, so should be relatively rare.

Test: N/A

Bug: 263411625
Change-Id: Ie0d79d66cf2b480e8d94c7d6b1567d7785cde7be