Get rid of unnecessary grant file locking that may potentially affect AM...

Get rid of unnecessary grant file locking that may potentially affect AM performance. Even though this file is accessed from separate threads, there is no space for race condition here because it is read in the very early stages of system boot but writes may only happen in 10 seconds after communication.

Bug: 149370983
Test: locally by flashing device and check basic functionality and atest UriGrantsManagerServiceTest
Change-Id: I3737646cbc0333b0dcbdc89fb8e165b0c2432018