ANDROID: fix a bug in quota2

If quota is precisely equal to skb->len then a notification
would not be sent due to immediately hitting 0.

This fixes that, and takes the opportunity to slightly clean
up the code and make quota behave more correctly for packet mode
as well.

Test: builds, net tests continue to pass
Bug: 164336990
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I78a11b48794496255513a6226c0469d809d7aa56