Add support for read-only SharedFileRegion

A receiver of a SharedFileRegion needs to know whether it is allowed
to write into it. Added a flag in the AIDL file to indicate that and
made this compatible with the READ_ONLY flag of IMemoryHeap.

Test: atest shmemTest
Change-Id: I3f26a36e152c2e9bf3aac4ff7f0067f9242b565b