[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
diff --git a/services/surfaceflinger/common/FlagManager.cpp b/services/surfaceflinger/common/FlagManager.cpp
index 3b669c6..3c2ccbc 100644
--- a/services/surfaceflinger/common/FlagManager.cpp
+++ b/services/surfaceflinger/common/FlagManager.cpp
@@ -210,7 +210,7 @@
 FLAG_MANAGER_READ_ONLY_FLAG(display_protected, "")
 FLAG_MANAGER_READ_ONLY_FLAG(fp16_client_target, "debug.sf.fp16_client_target")
 FLAG_MANAGER_READ_ONLY_FLAG(game_default_frame_rate, "")
-FLAG_MANAGER_READ_ONLY_FLAG(enable_layer_command_batching, "")
+FLAG_MANAGER_READ_ONLY_FLAG(enable_layer_command_batching, "debug.sf.enable_layer_command_batching")
 FLAG_MANAGER_READ_ONLY_FLAG(screenshot_fence_preservation, "debug.sf.screenshot_fence_preservation")
 FLAG_MANAGER_READ_ONLY_FLAG(vulkan_renderengine, "debug.renderengine.vulkan")
 FLAG_MANAGER_READ_ONLY_FLAG(renderable_buffer_usage, "")