Raise Handler ThreadPrority for IMMS

IMMS used the main thread with default priority
to create own Handler to deal with picker showing,
surface removal, and update input method list, etc.

Improve it by creating a dedicated thread with the
"THREAD_PRIORITY_FOREGROUND" priority to decouple
IMMS#mHandler from other task executions on the main
thread of system_server and reduce the likelihood of
priority inversion in IMMS.

Bug: 190486491
Bug: 167948374
Test: atest CtsInputMethodTestCases
Change-Id: Ia85df9c3e8b26f6fcbeaf269d47e929f178c9285