AArch64: Use long for pointers in BitmapRegionDecoder

For storing pointers, long is used in BitmapRegionDecoder
class, 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)

In addition, Graphics.cpp has been changed to work
with modified BitmapRegionDecoder.

Change-Id: Id54087dd3bfb29577e8b762e70946793369dc701
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>