Specify temp-allowlist reasonCode while calling addPowerSaveTempWhitelistApp().

Deprecate the overloaded addPowerSaveTempWhitelistApp() version that has
no reasonCode.

Bug: 171305836
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java
Reboot device, observe "tempAllowListReason" in logcat.

Change-Id: Id8ce4a2ac029b0cf33b2787f601d798f046bf2be
BYPASS_INCLUSIVE_LANGUAGE_REASON=Existing public API.