Create new mojo API for Always-on VPN feature

To expand the Always-on VPN feature on Android to apply
to chrome, we need to know what VPN package to apply it to.
Then we store that information in a pref in chrome, so
that it is available after restart, even before ARC++
has started.

BUG=b:72861260
TEST=unit tests

Change-Id: I2096acb14f74316186a83e76547bd6e9d0c116b7
Reviewed-on: https://chromium-review.googlesource.com/1052507
Reviewed-by: Mattias Nissler <[email protected]>
Reviewed-by: Luis Hector Chavez <[email protected]>
Commit-Queue: Philipp Weiß <[email protected]>
Cr-Commit-Position: refs/heads/master@{#579070}
8 files changed