Resolve orientation-behind for non-top visible activity

Assume that a task contains 4 activities:
 C translucent orientation:(any)
 B translucent orientation:behind
 A opaque orientation:landscape

After launching C, B satisfies the condition of non-top visible activity.
The orientation of B should be considered as landscape. Otherwise
DC#rotationForOrientation won't handle "behind" and return rotation 0,
which causes B to apply fixed portrait orientation unexpectedly.

Fix: 387935090
Flag: EXEMPT bugfix
Test: atest DisplayContentTests.testOrientationBehind
Change-Id: I00b22792551ab9efecc284aca9e2425efdb67fa0