Layout changes in v2 Network & internet page

This adds the skeleton of a header section which will only appear when
the device has two or more active mobile subscriptions. Subsequent CL's
will add the actual logic implementing it.

Also this moves up the airplane mode toggle higher up on the page, and
removes the proxy selector entry (which has been unconditionally
disabled by the ProxyPreferenceController for a while).

Bug: 116349402
Test: manual (this is mostly a layout change - subsequent changes will
have actual logic to test for the subscriptions header)

Change-Id: Iaee338d5c13e05d3c2d9d04872a94cf90e5da588