Don't recompute display mode during a mode switch

When an app requests a display mode change, it's plausible that the next
thing to happen is going to be a configuration change.

During the configuration change, multiple windows that contributed to
the selection of the display mode will no longer be displayed. This can
lead to a device bouncing between two display modes for a while.

We need to wait for windows to redraw and for the screen to unfreeze
again before we next call DisplayManagerService.setDisplayProperties.

Test: atest android.display.cts.DisplayTest
Bug: 158316271
Change-Id: I58e4626e74a911a14cbcd426d1f09ef68303dafa