lineage-sdk: Add Network Traffic [2/3]

Changes made since the original implementation from older branches:

*) Forward-ported to Oreo and adapted to Lineage SDK (bgcngm).

*) Implement LineageStatusBarItem interfaces to receive visibility
   and color tint information (bgcngm / sam3000).

*) Move from fw/b to lineage-sdk (sam3000).

*) Increase refresh interval from 1s to 2s (sam3000).

*) Don't generate messages when the statusbar isn't visible (sam3000).

*) Allow for choice of unit to be kb/s or Mb/s (and kB/s and MB/s). This
   deprecates threshold selection for autohide (sam3000).

*) Add an option for whether units should be shown in the
   statusbar (sam3000).

*) Various other simplifications (sam3000).

*) Added vector drawables (courtesy of kover).

Change-Id: Ia5aadc3f03a7b434a047accbd7d53f4aa44c77fb