Do not use last app rotation as default.

If the rotation sensor has been disabled we were substituting the
last app rotation for the sensor value. This fix uses the last
sensor value delivered before the sensor was disabled. Only use
the last app rotation if we never have received a valid sensor
value.

Fixes bug 6387946.

Change-Id: I50743c30ee2b4455e9848d3a619809be97eec3c8