Fix potential ApfFilter bugs by careful ByteBuffer use

Avoid adjusting ApfFilter.Ra.mPacket's postion() and limit()
in matches(). This avoids potential bugs in other parts of
the code that previously relied on limit() being reset.
Also for good measure change some limit() calls to capacity()
as it's more final.

Change-Id: I466e87ce6838f68654b24f2c9543a6cd547d3f87