Added shell command to check if device uses headless system user mode

It will be used by ITestDevice.

Test: adb shell cmd user is-headless-system-user-mode
Test: adb shell cmd user is-headless-system-user-mode -v

Bug: 174775905

Change-Id: Iee0a822fed69e32142efd14122bf0d1851a276a7
1 file changed