AArch64: Make graphics classes 64-bit compatible

Changes in this patch include

[x] Long is used to store native pointers 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)

[x] AssetAtlasManager is not completely 64-bit compatible
    yet. Specifically mAtlasMap member has to be converted
    to hold native pointer using long. Added a TODO to
    AssetAtlasManager.java to indicate the change required.

Change-Id: I940433f601c6db998c1a8ffff338f5361200d5ed
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>
Signed-off-by: Craig Barber <craig.barber@arm.com>
Signed-off-by: Kévin PETIT <kevin.petit@arm.com>
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>