AArch64: Use long for pointers in android/os/MemoryFile

Long is used in android/os/MemoryFile class to store
pointers as native pointers can be 64-bit.

In addition, some minor changes have been done
to conform with standard JNI practice (e.g. use
of jint instead of int in JNI function prototypes)

Change-Id: I07afc010524c16b5c852273b89becb0c08ff27d7
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>
Signed-off-by: Kévin PETIT <kevin.petit@arm.com>