Add explicit SYSTEM_UID check

to ActivityStarter#isUidPersistentSystemProcess()

Due to b/123502026 we're seeing some flakyness with
notifications especially, and SYSTEM_UID should never
be in those states.

Bug: 123359226
Bug: 110956953
Test: atest WmTests:ActivityStarterTests
Change-Id: I0d3c3b31c34cf4c9fa14e17ee10fc4b9b35553c7