AArch64: Use long for pointers in graphics/Interpolator

For storing pointers, long is used in
android/graphics/Interpolator 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)

Change-Id: I1e181476a4fe5273ff190cf34c4a7487aa1aecf4
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>
Signed-off-by: Kévin PETIT <kevin.petit@arm.com>