Bug 888150 - [MMS][Regression] Header actions are not working properly.

- STR1 Fixed
- STR2 Cannot Reproduce (https://www.dropbox.com/sh/3lr5a5kepnns8z1/rydxqmpucg)
- STR3 Fixed
- Fixes broken activity handling introduced here: https://github.com/mozilla-b2g/gaia/commit/8b41ee9a80c1d6b3613339e7388035d3fbcc575d#L2R227 includes tests.

https://bugzilla.mozilla.org/show_bug.cgi?id=888150
Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Merge pull request #10771 from rwldrn/888150

Bug 888150 - [MMS][Regression] Header actions are not working properly.