OomAdjuster: demote previous process to cached after configurable timeout

Previous processes shouldn't stay out of cached indefinitely; they
should be eventually moved to cached and allowed to be restricted and
frozen. Add a configurable timeout to decide when the previous app
should move to cached.

The default timeout is 60s.

Bug: 261065790
Test: atest MockingOomAdjusterTests

Change-Id: I2a999bbf5915467343441afcf276437e34459051