Put stopping service's host process on top of cached process list

stopService should be regarded as a service activity, thus after
the service gets stopped, its host process should stay on top of
all cached processes if it slips into cached state.

Bug: 162811773
Test: atest CtsAppTestCases:ActivityManagerTest
Change-Id: I4f42344c373e431b7b4418304f29e79c3dec1a0e