TempFSFile: Keep the WeakMap alive

If you store a WeakMap in the object, containing only circular
references in the values, then nothing is keeping the WeakMap alive and
it can be deleted along with its parent TempFSFile by the garbage
collector.

A WeakMap contains normal references to its values, so those normal
references can be marked by the GC like any other references.

Instead, store the WeakMap as a static property.

Bug: T332461
Bug: T332397
Change-Id: Idbaa1019c2bac3d1dd9b001753c2f4c0aaf66e98
(cherry picked from commit 7775a3f7b7df0d79097659941847726bc4c6a627)