AArch64: Use long for pointers in DRM classes

For storing pointers, long is used in DRM classes,
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: I101b0ea19ed93651c6ad1a9fa55e14e8be117469
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>