AArch64: Use long for pointers in DisplayEventReceiver

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: I3c0855373c0e4bedc172adb82b103586de9219dc
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>