Attempt to make data saver mode work for real.

The data saver refactoring change was incorrect in >= two ways:

1. It relied on the bw_costly_shared chain, which is currently
   unused. NetworkManagementService just has a "TODO: support
   quota shared across interfaces" comment about it. What
   actually happens when setting quota is that each costly
   interface chain (e.g., bw_costly_rmnet_data0) directly hooks
   in the bw_penalty box chain.

2. Implementing app whitelisting using "RETURN" inside
   bw_happy_box was pointless because if data saver was enabled,
   there was a REJECT at the end of the bw_costly_shared chain
   that it was returning to.

Instead, go back to the previous approach which hooked
bw_happy_box at the end of bw_penalty_box. Also, add an
additional bw_data_saver rule at the end of bw_happy_box.
bw_data_saver only contains one rule: RETURN if data saver is
enabled or REJECT if data saver is disabled.

That way:

1. If the app is blacklisted, bw_penalty_box REJECTs. If not:
2. If the app is whitelisted (system apps are always whitelisted)
   bw_happy_box RETURNs to bw_costly_rmnet_data0, skipping
   bw_data_saver.
3. If an app is neither blacklisted nor whitelisted, bw_happy_box
   jumps to bw_data_saver. If data saver is enabled, it REJECTs
   the packet, and if not, it RETURNs to bw_costly_rmnet_data0.
4. When we RETURN to bw_costly_rmnet_data0, either because the
   app is whitelisted, or because data saver is off,
   bw_costly_rmnet_data0 applies mobile data usage limits,
   and then RETURNs to bw_OUTPUT, which calls xt_qtaguid, etc.

Bug: 26685616
Bug: 27506285
Change-Id: If15397afde6862d95827a1fdd30f60efd7fab66a