Enable to override level by adb command for testing

This CL enables to overrde vendor API level by "adb shell setprop"
command, which is easier for testing among different API levels.

Test: manual
        1. adb shell setprop persist.wm.debug.ext_version_override 6
	2. kill demo app in recent apps
	3. launch overlay demo app
	4. observe the overlay container can be launched
Bug: 243518738
Flag: TEST_ONLY
Change-Id: I8d45a47b83e5044deb3606dbd580ffb6c444bf7a