Fixes collection modified while iterating by cloning the collection.

During a user switch, in MediaRouterService, a collection of users was
modified while iterating, which ends up skipping some elements of the
collection unintentionally. This CL creates a clone of the collection
before the for loop to prevent the issue.

Bug: 260410380
Test: n/a
Change-Id: Iec4d460ba0a0ca1325506e729f58715bb2d68898