Add shell command to control display windowing mode

This CL adds get/set-display-windowing-mode command to control the
display windowing mode.

Bug: 369954261
Test: manual - adb shell wm get-display-windowing-mode
Test: manual - adb shell wm set-display-windowing-mode 5
Flag: EXEMPT debug command
Change-Id: I704d44959d2dac081c58bbd14280648d9988f040