Add no-arg constructor to GC listener

Adding GC preparer as a listener would fail due to the lack of a
constructor with no arguments. This fixes that and also makes the hlper
final.

Bug: 119048359
Test: adb shell am instrument -w -r -e class
android.platform.test.scenario.notification.NotificationPressureMemorySuite
-e listener android.device.preparers.GarbageCollectionPreparer -e
garbagecollection-process-names com.android.systemui
android.platform.test.scenario/androidx.test.runner.AndroidJUnitRunner

Change-Id: Iea2ffe1818592a6bd067c64ec0549fa7c1a7e89f