Move ZramMaintenance to com.android.server.memory package

classes in services/core/java/com/android/server should be in the same
structure as the tests.

This is to align ZramMaintenance.java and ZramMaintenanceTest.kt in the
same structure.

We don't change ZramWriteback.java for a backward compatibility.

Bug: 375432472
Test: m && atest FrameworksServicesTests:ZramMaintenanceTest

Change-Id: Ib1eb07e388ffe010bfa1e62349b05ea6fc02d63d