AArch64: Make AssetManager and related classes 64-bit compatible

Following changes have been done:

[x] Long is used to store native pointers as pointers can be
    64-bit.

[x] AssetManager openAsset native function returned -1 if
    file name was empty and java function considered any
    non-zero value as success. This has been fixed by native
    function throwing Illegal Argument Exception as well.

[x] AssetManager incRefsLocked and decRefsLocked now accept
    long as input to support 64-bit native references.

[x] AssetManager incRefsLocked method incorrecly used
    'this.hashCode()' instead of the passed parameter id.
    This has been fixed.

[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: I095b9f900d49e51f43ad6afc47cbc23116a6a64a
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>