Determine whether to support activities in multi window (2/n)

Replace some existing usages of mSupportsNonResizableMultiWindow with
checking supportsMultiWindow() per Task/Activity.

This is cleanup before updating supportsMultiWindow().

Bug: 176061101
Test: pass existing
Change-Id: I3de46d6ecaae1ad93d031384e9cba6e0e4ebb01d
4 files changed