Show confirmation dialog before discarding drafts.

1. This CL implements the most common case, which is pressing the discard
  button on MessageCompose.

2. This CL doesn't change the discard on message list, which is much trickier.
   (We need to check the message type of all messages in the selection,
   as selection can contain any number of any kind of messges in "All Starred")

Bug 3072556

Change-Id: Ib270cae726f8fd5abf7e0e7dfc34ce97019d5301
1 file changed