Fix missed call notification dialing wrong number

This CL fixes a bug where clicking the "callback" button on the missed
call notification would dial the previous missed call.

To reproduce the bug you had to do the following:
    - get a missed call from Party A
    - clear the missed call count by launching dialer and navigating to
      the call log then pressing home. Clearing the missed call count by
      swipping away the notification should also work.
    - get a missed call from Party B
    - click "callback" on the new missed call notification Previously
      this would result in Party A getting called back.

The problem was that we were using the wrong pending intent flags:
- Flag 0: before ag/925201 we were using 0 which meant that the pending
  intent with Party A was being used.
- FLAG_ONE_SHOT: after ag/925201 we were using FLAG_ONE_SHOT which
  meant that the pending intent would be cancelled after the first use
  (after the user clicked callback on Party A). If the user never
  clicked callback on Party A then the intent would be re-used.

Fix was to use FLAG_UPDATE_CURRENT instead. This ensured that the
pending intent was always updated with the last missed call number.

Bug: 29065901
Change-Id: I8515e82d178348235d775b7a110e662652b3385b