Release AssetManagers when ejecting storage.

When ejecting a storage device, the system process needs to rapidly
release any open FDs to prevent itself from being killed by vold.

This change examines all ResourceImpls cached inside the system
process and evicts any that reference the storage device being
ejected.  (ResourcesManager will gladly recreate any evicted entries
when asked again in the future.)

Also replace broken use of WeakHashMap, since we want the values to
be weak references, not the keys.

Bug: 28867548
Change-Id: Ib9cfc66497149b6d3f8d49213e9779408a331d2a