Use Flags and SystemProperties on debug builds.

This will allow us to turn the feature on and off using `adb` commands
without rebuilding.

We'll limit this to debug builds to be safe.

After this CL, the feature can be enabled with:

```
adb shell setprop \
persist.sys.com.android.server.display.feature.flags.enable_connected_display_management \
true && adb reboot
```

Test: Manually tested that changing the flag enable the feature
Bug: 297159910
Change-Id: I5bcc03183cef047ae4ed3c1c0fff9450bbe996dd