(in preparation for moving it into netbpfload)

The programs themselves (in bpf_progs/block.c) required a 5.4+ kernel.

We relax this restriction to 4.19+ as we don't have any 5.4 device coverage
(while the pixel 4a 5G / 5 / 5a are all 4.19 devices).

I believe we could relax it further to 4.14+ but Pixel 4/4xl/4a that
would exercise those code paths are EOL and probably have poor to
non existent test coverage, and we cannot do anything for 4.9 T devices
anyway.

Note: on <4.19 kernels (ie. T devices running 4.9/4.14, U running 4.14)
this results in ConnectivityNativeService going from null to initialized
(as the bpf map will exist).

This doesn't hurt as the set/clear port interfaces are only ever
called by vendor code on devices where the kernel doesn't support
the older mechanism.  And even if you call them it will just set/clear
the bits in the bpf bitmap, they just won't actually affect anything.

We could flag the map itself as being 4.19+ as well, but I think
I prefer the no-op map to exist...

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I1085addd22f4f3b709e1875049633832c5dac836