ApfFilter: systematically use u8, u16, u32 getters

This patch adds a getUint8 getter for ByteBuffers and changes ApfFilter
to make uses of getUint8/16/32 everywhere.

The return types of getUint16 is also changed from long to int, which
will expand gracefully to long as an unsigned int as it is guaranteed to
be positive after getUint16.

Test: ApfTest passes
Change-Id: Id3c2817b949e9817e2bb60e0ec8fe2d6fd8d787c