InCallUI: Disable Back button on Non-intursive InCall Card

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

PS2: code cleanup

PS3: Remove unwanted space and proper intendation

PS4: again code formatting

PS5: Nobody likes tabs

Change-Id: I257daccbb175265e2430f1b0b30c9175cb4b1981

Patchset: 5
https://gerrit.omnirom.org/#/c/4555/