Fix race condition between Looper and destructor

1. Clean all messages before add new.
2. Insteading of using `this`, use the unique mStaleHandler sp so Looper
   can hold the sp to keep the instance alive until the last message
   done.

Test: Manual Test
Bug: 219965773
Change-Id: Ic039146f0b966c1f27d86b121d4b72b75ff360e5
2 files changed