Added more info on dumpsys device_config.

Example:

$ m sync && adevice update && adb shell dumpsys device_config |head -3
SyncDisabledForTests: none
Is mainline: false
DeviceConfig flags:

Fixes: 364399200
Test: see above
Flag: TEST_ONLY

Change-Id: Ifab219bccededb07de558dced9d6203efe568609