InCallUI: fix non intrusive incallui layout

- makes the decline/answer buttons evenly sized
- decrease font size of buttons to 13sp so the non-english button labels fit better

before: https://www.dropbox.com/s/g9mn72qhzq6wlbt/2014-01-01%2006.00.31.png
after:  https://www.dropbox.com/s/dfhmaliz6a21lk5/2014-01-01%2010.30.42.png

Change-Id: I88e0f0a1358595f2ce86dddcae2ab4d4e788fc1a
InCallUI: Disable Back button on Non-intursive InCall Card

Fix for jira issue http://jira.omnirom.org/browse/OMNI-296

PS2 to PS5: code cleanup

PS6: Remove Jira issue url from code

Change-Id: I257daccbb175265e2430f1b0b30c9175cb4b1981