BroadcastQueue: fix subtle OOM adjustment bug.

updateOomAdjPendingTargetsLocked() only takes effect for processes
that are already running as warm; it doesn't work on a process that
is still being cold started.

This change fixes the bug by making an explicit update call once
a pending cold start has become a warm process.

Bug: 255985948
Test: atest CtsAppTestCases:ActivityManagerProcessStateTest#testBackgroundCheckBroadcastService
Change-Id: I7e42984c7b7fd6e496fc9245304f1eff8ad71075