AArch64: Make AssetAtlasService 64-bit compatible

Changes in this patch include

[x] Long(64-bit) is used to store native pointers in
    AssetAtlasService and related classes as they can be 64-bit.

[x] 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: Ib4c77c134e3ad5b21732e20cde9a54a0b16bdab1
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>