Make VideoSourceImpl keep track of the started device

Previously, when PushSubscription is created for an already started device, the subscription would be created with no device set. Now VideoSourceImpl will keep store a pointer to the started device until it's stopped, and will use this pointer to configure newly created PushSubscriptions.

Bug: 1449684
Change-Id: I6d3c9e8df413cdaad6229eab48467836bb46b811
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4571364


Commit-Queue: Sean Li <seannli@google.com>
Reviewed-by: Ilya Nikolaevskiy <ilnik@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1152633}