AArch64: Add AssetInputStream.getNativeAsset

This patch adds AssetInputStream.getNativeAsset. This function
returns asset pointer, which can then be passed to JNI functions.
Note that this function is hidden and not part of Android API.

This function will eventually replace getAssetInt,
once all the classes that use getAssetInt are changed
to use getNativeAsset instead.

Change-Id: I2a2b2e6864f1f57ae2b3a10c45559b3635266ed5
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>