Process adj could stay at UNKNOWN_ADJ till next full adj update

Fix an issue that a non-cached process goes to cached state, but
there is no other connections to this app, its adj value could
be UNKNOWN_ADJ, till the next full adj update occurs.

Bug: 135941673
Test: atest MockingOomAdjusterTests
Change-Id: I397b26c0b4c2b84a8c418cc21f6e0c85950e39e7