Break MessageViewFragment up into two fragments, MessageViewFragment, which
is used to show regular messages, and MessageFileViewFragment, which shows
EML messages. (And their base class, MessageViewFragmentBase.)
MessageViewFragmentBase's javadoc has a class diagram.
MessageViewFragment is actually named MessageViewFragment2 at this point
so that GIT correctly finds out the rename from MessageViewFragment to
MessageViewFragmentBase. I'll rename it back in a following CL.
Also added very basic unit tests for MessageView and MessageFileView.
At this point, they just make sure the activities really open and show
messages without exceptions.
I feel like the current naming schema for the activities/fragments is
kinda confusing. Let me know if you come up with better names.
Change-Id: Iff948f4b68cfdb7c1e68f225927b0ce58d34766b
