Which applications the tunnel carries belongs with the rest of a
profile's preferences rather than on the Android side, so the choice
follows the profile the user is on.
Adds a split tunnel store beside the SSH session store, over a
"split-tunnel" namespace holding the mode and both selections. The two
selections are kept apart because the platform applies an allow list or
a deny list and never both, and so that switching mode does not throw
away the picks made in the other one.
Only the store itself needs the android build tag; the rest stays
untagged so it is covered by the package's host tests.
