Fix race condition in OomAdjuster.

OomAdjuster accesses the not-thread-safe HandlerThread.getThreadId from
outside the thread.
Modify so it posts a message to thread instead.

See bug 22476742

Bug: 151407445
Test: See repro instructions in bug
Change-Id: Iae03d46896561f0442af6595a8d0bdf617ccd61e