config: add stub for ActivityManager.addOnUidImportanceListener()

It requires the special access PACKAGE_USAGE_STATS ("Usage access") permission that Play services
usually doesn't have.

removeOnUidImportanceListener() throws an IllegalArgumentException when trying to unregister an
unknown listener, but this is not an issue: app-side mImportanceListeners map is still updated
in addOnUidImportanceListener(), despite the listener registration being skipped.

gmscompat_config

@@ -70,6 +70,9 @@ finsky.kill_switch_phenotype_gservices_check set-string 0
 
 [[stubs]]
 
+[android.app.ActivityManager]
+addOnUidImportanceListener void
+
 [android.app.admin.DevicePolicyManager]
 isDeviceProvisioned true
 isDeviceProvisioningConfigApplied true