PerftestMultiuserDummyApp > MultiUserPerfDummyApp

Renames PerftestMultiuserDummyApp to MultiUserPerfDummyApp.
This dummy app will be added to the platform testing, and it's useful
for the name of the dummy app to be alphabetically adjacent to the
test app (MultiUserPerfTests).

Also removes an unnecessary Android.mk file in an empty directory.

Bug: 133510429
Test: atest multiuser.UserLifecycleTests#managedProfileUnlockAndLaunchApp
Change-Id: Ifcdaa36e39b6d2a5e42decd9277d9afbf3a75d78