Don't collect all reverse reachable processes on oom update

Only the immediate clients of the target processes are needed when
performing a partial update, since the clients not in the target process
list will not change. This means their set procState and oomAdj can be
trusted to represent all the reverse reachable process.
Also don't bother handling cycles, all clients that are part of a cycle
with target processes will already be in the target process list.

Test: atest MockingOomAdjusterTests
Bug: 311472557
Change-Id: I7f4387f84c166f06cc3cf8c2bf0b0773415c0f5e