Clear the notifications map when notifications are dismissed.

Uses a broadcast receiver that notifies when notifications are cleared/swiped away in the notifications area.

Fixes #1481
Use context already passed in `onMessage`
Changed notifications map to be private and final.

Also fixed a few code style issues in `GCMIntentService` that I noticed.
Merge pull request #1495 from wordpress-mobile/issue/1481-push-dismiss

Clear the notifications map when notifications are dismissed.