Enforce all the SliceBackgroundWorkers being singletons at syntax level

- Create workers via reflection in SliceBackgroundWorker
- Store the workers in a static container and release then at shutdown()

Fixes: 118228009
Test: robolectric
Change-Id: I564277d3a12b2d7d3b50cef091bdfedb3397c145

proguard.flags

@@ -60,4 +60,9 @@
 # Keep classes that implements CustomSliceable, which are used by reflection.
 -keepclasseswithmembers class * implements com.android.settings.slices.CustomSliceable {
     public <init>(android.content.Context);
-}
+}
+
+# Keep classes that extends SliceBackgroundWorker, which are used by reflection.
+-keepclasseswithmembers class * extends com.android.settings.slices.SliceBackgroundWorker {
+    public <init>(android.content.Context, android.net.Uri);
+}