MediaRouter: Enable background dim for dialogs
MediaRouter: Enable background dim for dialogs

Bug: 64924925
Test: manual using SupportV7Demos
Change-Id: I75417381068ebb5158e8e3ba48e4db89bfcb303f

aosp-mirror/platform_frameworks_support,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,aosp-mirror/platform_frameworks_support,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,AndroidX/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx