Hide tab when disabled, metered strings, fixes.

Hide "Data usage" item when kernel support isn't available and
enabled.  Change "restrict background" strings to reflect metered
status.  Hide app settings button when no package found.  Always
start limit sweep above any warning.  Imply metered state for
networks with limits.

Bug: 6303364, 5465643, 5379686, 6376276, 6368329, 6376276
Change-Id: I4faccc9a50f2f741fd937bb8c361a3ca055135e2