Synchronize before calling unstableRemoveIf

The call to unstableRemoveIf should be synchronized on the
ResourcesManager instance since the collection belongs to the instance.

Also remove dead code from the loader refactor to prevent apps from
creating AssetManagers that return InputStreams instead of
AssetInputStreams.

Bug: 145510133
Test: boots
Change-Id: I50bcfc8e882f3d8a7827688e32f68790dd226801