DtlsInStun: Modify termination by application data

This is a follow up to https://webrtc-review.git.corp.google.com/c/src/+/45152
which I think(?) address the comment in
https://webrtc-review.git.corp.google.com/c/src/+/451521/comment/0e8a1639_699fdc3a

I.e. encrypted srtp is only pass though by the dtls layer, meaning we
need to check if we are writable before concluding that both are
writable.

Update method name to ApplicationPacketReceived to highlight
that it doesn't have to be decrypted.

Bug: webrtc:367395350
Change-Id: I37f2d0c2fdeb6cb33c1a042b30d2afdfe1524032
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/452280
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Sameer Vijaykar <samvi@google.com>
Cr-Commit-Position: refs/heads/main@{#47000}