Improved the heads up experience when notifications were blocked

When notifications were blocked due to proximity, the heads
up would never show until unlocked. We're now showing it
when the screen turns on.
This also avoids the header to be shown while the
notifications are fully collapsed, since that was the
only element that changed before.

Fixes: 	137711785
Test: get hun in pocket, pull out, observe notification visible
Change-Id: I9c3d0cdc341eb2d95bcb76099e59b0d561f81210