* Turns out that Exchange 2003 requires the ics attachment to be in a
multipart/alternative, rather than a multipart/mixed MIME message
* Exchange 2007 accepts both types
* Therefore, we change our output for this particular situation, i.e.
a single attachment that is an ics file, to multipart/alternative
* Rename FLAG_SUPPRESS_CONTENT_DISPOSITION to FLAG_ICS_ALTERNATIVE_PART
and make this flag do double duty - 1) suppress the Content-Disposition
header (also required by Exchange) and 2) send the message as
multipart/alternative
* Add unit tests for Rfc822Output to check that mime parts are composed
properly
Bug: 2516394
Change-Id: I60e26f57b8ecaf01d0340e7828533334e0e7d45a
0ed690bfb4