Introducing MessageListFragment.

- Extracted MessageListFragment out of the MessageList activity.
- This is basically pure extraction, with the following conceptual change.
  - Now the MessageList activity doesn't know the mailbox id or
    the account id.  If it needs these ids, it needs to ask the fragment.
  - MessageListFragment.LoadMessagesTask tries to determine the account ID
    if it's unknown.

Most code in MessageListFragment is directly copied from MessageList
with minimal changes (e.g. pass mActivity instead of 'this' as a Context).
There's a few cleaning up oppotunities.  I'll work on them later in a separate
CL.

Change-Id: Ie004cc49b429f2cd8f9de73df5abb94f3054ea0a
Merge "Introducing MessageListFragment."
Use Fragment's context menu API.

Change-Id: I1be6b17ff5bc7fd5258366bc064251c6f79c0cc8
Merge commit '2aa8ea4399dd70c26d6c0679ff002fb16fdc5bff' into oxygen_1006

* commit '2aa8ea4399dd70c26d6c0679ff002fb16fdc5bff':
  Use Fragment's context menu API.
  Introducing MessageListFragment.

Conflicts:
	src/com/android/email/activity/MessageList.java