Move flag feature_overrides to namespace "gpu"

Move the aconfig feature flag "feature_overrides" from the
"core_graphics" namespace to "gpu".

Note: This necessitates a new flag name as well:

go/aconfig-namespace#aconfig-namespaces-2
> Once a flag's declaration is merged, its namespace cannot be changed.
> If you've assigned the incorrect namespace, rename the flag and assign
> the correct namespace in the same change.

Due to this, the name is being changed to "angle_feature_overrides".

Old:
  com.android.graphics.graphicsenv.flags.feature_overrides
New:
  com.android.graphics.graphicsenv.flags.angle_feature_overrides

Bug: 372694741
Test: aflags list | grep angle_feature_overrides
Test: aflags enable com.android.graphics.graphicsenv.flags.angle_feature_overrides
Flag: com.android.graphics.graphicsenv.flags.angle_feature_overrides
Change-Id: Idc859b76b846901e1e763c4a994c98d050b2b4b9