Move the app exit handler to a new thread

Previously the handler was sharing the same thread with LmkdConnection
which takes care of receiving incoming data from lmkd; in case there
is any lock contention, the lmkd communication would get wedged.

Bug: 148387160
Test: atest ApplicationExitInfoTest
Test: atest CtsAppExitTestCases:ActivityManagerAppExitInfoTest
Change-Id: Ifc0aacec0e64e1d920ae61e65eef94ea9e3b1828