[Cast MRP] Split cast_activity_manager.{cc,h} apart.

Bug: 954797
Change-Id: Idec1306428c4efa4500534ad970da1606e132003
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575435
Commit-Queue: John Williams <jrw@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653361}
8 files changed