Ignore flow label when comparing RAs in APF

This change modifies the RA filter generation code to ignore bytes 1-3
of the IPv6 header. It also updates testApfFilterRa such that it fails
without the updates to ApfFilter.

Test: as follows
        - built
        - flashed
        - booted
        - sent RAs from OS X with differing flow labels and observed
	      that they were filtered
	- ApfTest passes
Bug: 31166495

(cherry picked from commit 7006bd4fe5eba4b6fad43dd28d9cad2d26ce140a)

Change-Id: I32153238193ac13a0e59be1875d983a0567c41c7