Revert weak media provider (#4483)

* Revert "Address the real lifetime issue of the SDK's `Client` by making `Context.mediaProvider` weak. (#4466)"

This reverts commit b9d1558216.

* Better docs.

UnitTests/Sources/RoomChangeRolesScreenViewModelTests.swift

@@ -196,7 +196,7 @@ class RoomChangeRolesScreenViewModelTests: XCTestCase {
         roomProxy = JoinedRoomProxyMock(.init(members: .allMembersAsAdmin))
         viewModel = RoomChangeRolesScreenViewModel(mode: mode,
                                                    roomProxy: roomProxy,
-                                                   mediaProvider: .mock,
+                                                   mediaProvider: MediaProviderMock(configuration: .init()),
                                                    userIndicatorController: UserIndicatorControllerMock(),
                                                    analytics: ServiceLocator.shared.analytics)
     }