Fixed how ContentCapture is kill-switched.

We provide a way for OEMs to kill-switch Content Capture, but it currently
does not work in the first boot (which uses default settings).

This CL changes the mechanism:

- If the property is not set, it assumes it's disabled (before it was only
  disabled when explicitly set to "false").
- To always enable it, it must be set to "always" (before it was "true").
- To check for the overlaid resource, it must be set to "default" (before it
  had to be unset).

Test: manual verification

Fixes: 121144410
Bug: 121153631

Change-Id: Ie669e43d9dce947a7bb31bc3b1768774f724675f