No crash from AHardwareBuffer_unlock on NULL fence

The API document states: "If fence is not NULL then it will be set to a
file descriptor that is signaled when all pending work on the buffer is
completed." As it's too late to change the document, we should probably
update the implement and use GraphicsBuffer::unlock under the hood.

Bug: 62302267
Test: Build, AHardware CTS
Change-Id: Ic0fac5d7addfba67baaa1b5d7c221fc25d02df56