Make default orientation of window container unset

The UNSPECIFIED is also a kind of provided orientation. If a
window container doesn't provide orientation, it should return
UNSET instead. As the implementation getOrientation of various
container, e.g. ActivityRecord, TaskFragment, DisplayArea, which
return UNSET if it can not decide the orientation. And the
UNSET will be reported as UNSPECIFIED in DisplayContent.

This could speed up a bit when traversing WindowContainer#
getOrientation that skips UNSET directly.

Bug: 163976519
Test: testGetOrientation_childSpecified
Change-Id: Iec067673e48dab559f5976f15d3e6524e0657b44