Opp: Group 'Sent/Received files' notifications using group summary

Bug: 323782678
Bug: 323096132
Test: m -j;
Test: Manual, after receiving and sending a file via bluetooth,
      and wait about 3 seconds until the grouping is handled by system.
      Now open the notification drawer, then the
      'Sent/Received files' notifications are grouped together.
Change-Id: I1a67355a34a7c253b9693a1eade89f86504e89d7