tempAllowList duration can be updated if an UID is already temp

allowlisted.

Previously when PowerExemptionManager.addToTemporaryAllowList() is
called for a second time before the first time call's duration expires, the
second call does not update ActivityManagerService's
FgsTempAllowList for the new duration. This CL fixes this problem.

Bug: 199801023
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java
Change-Id: Ibb8c6f41740a9b7468b50a4cdabe526ae58550ec