Fix process capabilities on re-computation for cycles in oomAdjuster

When retrying the oomAdj computation because of cycles, it's scanning
the list of processes in the opposite direction, we should preserve
whatever capabilities we have got before the retry.

Bug: 206585758
Test: atest MockingOomAdjusterTests
Change-Id: Ia7d0ec7a39aac5bf59ae837b4af1ed7b6512521c