[hwc-batching] Add option to override feature flag locally

For testing purposes, adding a way to switch flag on/off.
Bug: 290685621
Test: Ran following commands and checked if createLayer or the batched version is called.

% adb shell dumpsys SurfaceFlinger|grep -C25 "FlagManager values"
 enable_layer_command_batching: true
% adb shell stop && adb shell setprop debug.sf.enable_layer_command_batching false && adb shell start
% adb shell dumpsys SurfaceFlinger|grep -C25 "FlagManager values"
 enable_layer_command_batching: false

Change-Id: I47ac3859b35e41b6dcb31c23224692b8d75e8218