Add observer for WindowManager developer options

Some WM-related developer options write a setting entry, which is
initialized only at system boot. We need to reboot after the setting
chanaged. The CL register an observer to "Enable freeform windows",
"Force desktop mode" and "Force resizable activities". It will take
effect after setting changed without reboot.

Bug: 129279411
Test: A. atest WmTests:WindowManagerSettingsTests
      B. Manual test.
         1. Enable "Force desktop mode",Freefome window","Force resizable activities" from
	    developer option.
         2. connect mouse (USB or Bluetooth).
         3. Create simulated display from developer options.
         4. Launch activities from launcher on simulated display and interact with mouse.

Change-Id: I7b5e15f548de5c2f163a1816020fe46756968f5e