After we switch VDA to use MojoVideoDecoder, we should be able to remove CdmManager

CdmManager provides a singleton registry of CDM instances. This is used to share ContentDecryptionModules between the MojoMediaService and AndroidVideoDecodeAccelerator, and should be removed along with AVDA in the future after we switch to use MojoVideoDecoder.

Note that MojoCdmServiceContext serves the same purpose for Media Mojo services, but is scoped to a single InterfaceFactory.