Refactoring: Delete AppWindowToken.willBeHidden field.

The only time AppWindowToken.willBeHidden is used is for determining
if the app should contribute to calculating orientation. In the same
check AppWindowToken.hiddenRequested will be or-ed with willBeHiden,
so it's enough that hiddenRequested to be set.

The only place where willBeHidden is set, is right before
WMS.setAppVisibility is called, which will set hiddenRequested.
Because of this willBeHidden is unnecessary.

Change-Id: Iea35f39f72e7f0dcd76205ef580f3a74cac72d08