docs: Adding N preview docs on TV recording APIs

Adding docs on TV recording APIs, formerly known as DVR APIs.
Only focuses on APIs for channel apps. For playback apps, these
APIs should get documented by AOSP team for OEMs and partners.
Adding updates from CL reviews.

Bug: 27048027
Change-Id: I18ce62a3d0049a59a57f3daae71e26261688089d