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

services/core/java/com/android/server/am/OomAdjuster.java

@@ -227,7 +227,8 @@ public final class OomAdjuster {
         final ServiceThread adjusterThread =
                 new ServiceThread(TAG, TOP_APP_PRIORITY_BOOST, false /* allowIo */);
         adjusterThread.start();
-        Process.setThreadGroupAndCpuset(adjusterThread.getThreadId(), THREAD_GROUP_TOP_APP);
+        adjusterThread.getThreadHandler().post(() -> Process.setThreadGroupAndCpuset(
+                adjusterThread.getThreadId(), THREAD_GROUP_TOP_APP));
         return adjusterThread;
     }