tp,ui,docs: Surface the display-video user-build sysprop in docs, record page and trace doctor (#6717)

Screen recording (`android.display.video`) is now capturable on `user`
(production) builds, not just `userdebug`. On user builds it must be
unlocked
  first, over ADB, until the next reboot:                              
 
adb shell setprop debug.tracing_video_allowed true
                                                                       
The property is cleared on reboot, so it has to be re-set after each
restart
before display video can be captured. Once set, capture works through
any of the
existing methods (the on-device toggle, the record page, or a raw
config).
                                                                       
This makes that requirement discoverable in three places:
**Docs** (`docs/data-sources/video-frames.md`)
- Drop the "userdebug devices only" statement.
- Add a "Prerequisite on `user` builds" section documenting the
property, the
command, and its reset-on-reboot behaviour; point the intro/TOC at it.
  **Record page** (`Display video frames` probe)                       
- The probe description now spells out the user-build requirement and
shows the
`setprop` command as a code snippet, and links to the data-source doc.
                                                                       
  **Trace doctor** (new `display_video_not_enabled` diagnostic)        
- Fires when a trace requested `android.display.video` but captured no
frames,
on a `user` build, with no producer error stats — the usual cause being
the
    property was not set. 
- The video-frames table is owned by the `video_frame_importer` plugin
and isn't
reachable from the common diagnostics framework, so the plugin now bumps
a new
`android_video_frames_emitted` stat per inserted frame; the rule reads
emptiness through that stat, alongside the existing `android_video_*`
error
    stats.