Update the DM sysprop flags to override gantry's

In the DisplayManager, the sysprop flags are now suffixed with
`-override` and, if defined as true or false, will override the flag
from Gantry.

This allows testing what happens if the flag is turned on during
development or off during trunk staging or trunk fooding.

Fix: 304233664
Test: Manual
Change-Id: Iff5bd0aa8c99b0bb197701ac0f5f85af6bdc9510