Frameworks: Actually start to use HEADS_UP_REQUESTED extra

Prepairing slim heads up feature, we do this here to allow apps to
send a heads up notification no matter what.

Google introduced this flag but is not using it in the actual API

Take HEADS_UP_REQUESTED into account to indicate that this notification
is a good heads up candidate.

How to use it you can see in email or mms app

Change-Id: I4a29404122ae12ab9b95e1d223911078a360b44e
SystemUI: Status bar clock - fixes

This commit fixes:

- no overlapping more from status bar icons on center clock.
  The width depending on what the user has setuped (date and clock) is exactly meassured now
- do not start animations if they are not needed
- reduce settings provider calls
- a bit cleanup

Change-Id: I7a37a6ae8f9bad0f265119af05dc20cf21cd6890