[merge] media: Disallow CDM reset in mojo media services

In general, after playback starts, we do NOT support removing or
resetting the CDM if one is already set. There's no use case for this
at least for now and foreseeable future, and this change would help
simplify some logic.

(cherry picked from commit 7d25c6948d6e0fb55da2e8f637caa586f6c6be55
with modifications to resolve merge conflicts)

Tbr: jrummell@chromium.org, lukasza@chromium.org
Bug: 1066893
Change-Id: I8efce1f26c30c8cfc51c17ab4843869f2847894a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134922
Reviewed-by: Ben Mason <benmason@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/branch-heads/4044@{#894}
Cr-Branched-From: a6d9daf149a473ceea37f629c41d4527bf2055bd-refs/heads/master@{#737173}