Migrate ENABLE_GRID_ONLY_OVERVIEW to aconfig

- Keeping FeatureFlags.ENABLE_GRID_ONLY_OVERVIEW for local testing until aconfig is available in FlagFlipper
- Still reference ENABLE_GRID_ONLY_OVERVIEW in tests as aconfig still can't be mocked

Bug: 270397206
Test: manual
Change-Id: I799e05311adc21184a6ad7ce92a3abdab3f25c25
diff --git a/aconfig/launcher.aconfig b/aconfig/launcher.aconfig
index aca1b3b..226d717 100644
--- a/aconfig/launcher.aconfig
+++ b/aconfig/launcher.aconfig
@@ -13,3 +13,10 @@
     description: "Enables two line label inside all apps."
     bug: "270390937"
 }
+
+flag {
+    name: "enable_grid_only_overview"
+    namespace: "launcher"
+    description: "Enable a grid-only overview without a focused task."
+    bug: "270397206"
+}