Clean up logic for killing bg restricted apps

Clean up logic around killing cached processes in idle, background
restricted UIDs. ProcessStateRecord includes flags for marking a process
as exempt from this type of killing, but this functionality is not and
has never been used. Remove the flags that are always false to simplify
the code. Also, rename mLastCanKillOnBgRestrictedAndIdleTime to
mLastCachedTime, since that is what it's actually tracking.

Test: atest ActivityManagerTest#testKillAppIfBgRestrictedCachedIdle
Flag: EXEMPT refactor
Change-Id: I401661d4066404f44f17235114acddee311d29c3