Return a pointer of MappedStorageFile/MutableMappedStorageFile instead
of an object of MappedStorageFile/MutableMappedStorageFile. Now added
destructor for MappedStorageFile/MutableMappedStorageFile to unmap the
in memory file to free up memory.
Bug: b/321077378
Test: atest -c
Change-Id: Iaa02696feb07ff383f0c7e46b645d82e57c38254