AArch64: Correction to OMX_U32 and OMX_S32

The types of OMX_U32 and OMX_S32 were appropriate to a 32-bit system
but were not appropriate to a 64-bit system. They have been changed to
make them portable between the two systems. With this change and a
change to frameworks/av/media/libstagefright/foundation/AString.cpp,
the android.media.cts.DecoderTest#testFlush CTS test passes.

Change-Id: I8e16c7169f54599c23fea71585b531e48541c78a
Signed-off-by: Marcus Oakland <marcus.oakland@arm.com>