Look for SPS instead of keyframe with H264 packetization_mode=0

[sgfn]

As explained in the comments, checking for nalu_type == 7 with H264 packetization_mode=0 has been verified to work correctly under simulated packet loss conditions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.55%. Comparing base (9cdd430) to head (5a37aae).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   87.55%   87.55%           
=======================================
  Files          50       50           
  Lines        2684     2684           
=======================================
  Hits         2350     2350           
  Misses        334      334           

Files with missing lines	Coverage Δ
lib/ex_webrtc/peer_connection/configuration.ex	95.05% <100.00%> (ø)
lib/ex_webrtc/rtp/h264.ex	47.05% <100.00%> (ø)

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