FrameEncodeMetadataWriter: Don't make callbacks while holding lock.

The FrameEncodeMetadataWriter class holds state about pending frames.
Based on callbacks, it may determine the some frame have been dropped,
and will on those cases call the EncodedImageCallback::OnFrameDropped()
method.

This CL makes sure it doesn't hold the internal mutex while making such
callbacks, as that poses a risk for deadlocks.

Bug: webrtc:467444018
Change-Id: Ie31ec6d725cda95b44ec25889609c2df84e58730
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/458580
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47216}