Add flush() method to flush the data whenever needed

To sync the data accross processes, sometimes need to flush the data
manually. Add the method and put the I/O into background to prevent
trafiic in main thread.

Flag: EXEMPT bugfix
Bug: 357878944
Test: atest HearingDeviceLocalDataManagerTest
Change-Id: I0c096cfbdf240ed63af8abc80f4d6e102864ce8e