Add onUserStorageLocked to signal to Keystore

Signal to keystore from UserController when a user
has successfully evicted their CE keys.

Bug: b/376496265
Flag: com.android.server.flags.user_data_refactoring
Test: atest UserControllerTest w/setprop fw.stop_bg_users_on_switch -1
Change-Id: I49a56820c6b7cd576cd4f8f4c5bba16e671e4555