Remove remaining messages after Telephony tests end

After finishing a test, the existing handlers might not be freed because
they were still processing existing messages. Clear all remaining
messages in tearDown to prevent memory leaks.

Test: atest FrameworksTelephonyTests OOM before and passes after
Bug: 220401651
Change-Id: I9d5f5b50d2425ef044203351e15f428838213534
Merged-In: I9d5f5b50d2425ef044203351e15f428838213534