Add a process state seq counter to UidRecord.

This seq counter is associated with the process state in UidRecord
and will be incremented whenever the uid state is going from
background to foreground or vice versa.

Bug: 27803922
Test: runtest -c com.android.server.am.ActivityManagerServiceTest frameworks-services
Change-Id: I1183d929bc7e0b2c9912de3822eb344d2bb0dcf7