Migrate trim_resources_with_background_trim to aconfig flags

Use aconfig to manage this flag and continue rollout.

Bug: 322143614
Bug: 283071711
Test: unit tests
Flag: ACONFIG com.systemui.trimResourcesWithBackgroundTrim DISABLED
Change-Id: Icf665eb60658aaea397d804c5465714065ef1fdd
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index 994a020..a0dd922 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -509,3 +509,13 @@
         purpose: PURPOSE_BUGFIX
     }
 }
+
+flag {
+    name: "trim_resources_with_background_trim_at_lock"
+    namespace: "systemui"
+    description: "Trim fonts and other caches when the device locks to lower memory consumption."
+    bug: "322143614"
+    metadata {
+        purpose: PURPOSE_BUGFIX
+    }
+}