AArch64: Use long for pointers in services

For storing pointers, long is used, 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: Ib4435f0794740d545c1e640087849215e6844802
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>