Wrap some calls out of OomAdjuster with the injector

Small step to preparing OomAdjuster to be compartmentalized from the
rest of the system. Also update the tests to also validate OomAdjuster
actually tries to send oomscore updates to lmkd.

Flag: EXEMPTED bugfix
Test: atest MockingOomAdjuster
Fixes: 364792232
Bug: 346822474
Change-Id: I38817d64262d15b8e871979e3bd143520e570959