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ClearKey is a software based key system, we should not ask backend to
handle decryption with this key system.

The patch delays the creation of StreamDecryptor until we know which key
system to use.

BUG=internal b/117273176
TEST=cast https://simpl.info/eme/clearkey/

Change-Id: I5de56f84c52b42f5440928406a36ae364e61c1fc
Reviewed-on: https://chromium-review.googlesource.com/c/1284950Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600173}