Fix issue #5755172: Soft menu key disappears when menu is open

We need to work more like before in determining whether the menu
key is needed -- in some cases look back in the window list to
determine this if we don't know the value from the current window.

This requires adding a new private flag indicating whether the
compat menu state is known for a window, which is set by
PhoneWindow as part of its existing process of computing the flag
for its own windows.

Now we can have a new API on WindowState to determine the value
of this flag for a window, which if needed walks back in the window list
to find a window the value is known for (or stops at what the policy
has determined is the top full-screen window, so we stop like we used
to at things like the lock screen or the bottom of an application).

Change-Id: I829de6d629b5af8bcb422cb85249ee4041c7205e