FFmpeg software H.264 decoder: opt-in via pubspec flag

- Gate buildFfmpegJni + jniLibs packaging on `ux: enable_ffmpeg` in the
  consuming app's pubspec (default off) -- no LGPL / H.264-patent
  exposure unless explicitly enabled
- appInfoBuilder generates kUxEnableFfmpeg from the same flag so apps
  register the FFmpeg LGPL notice eagerly, pubspec-only (no dart-define)
- Add registerFfmpegLicense() + bundled LGPL-2.1 text asset
- FFmpeg compliance docs (LICENSES-3RDPARTY.md, android/ffmpeg/README.md)
- Network video streaming: XVideoPlayerController.network

example/pubspec.lock

@@ -343,10 +343,10 @@ packages:
     dependency: transitive
     description:
       name: matcher
-      sha256: dc0b7dc7651697ea4ff3e69ef44b0407ea32c487a39fff6a4004fa585e901861
+      sha256: "12956d0ad8390bbcc63ca2e1469c0619946ccb52809807067a7020d57e647aa6"
       url: "https://pub.dev"
     source: hosted
-    version: "0.12.19"
+    version: "0.12.18"
   material_color_utilities:
     dependency: transitive
     description:
@@ -484,10 +484,10 @@ packages:
     dependency: transitive
     description:
       name: test_api
-      sha256: "8161c84903fd860b26bfdefb7963b3f0b68fee7adea0f59ef805ecca346f0c7a"
+      sha256: "93167629bfc610f71560ab9312acdda4959de4df6fac7492c89ff0d3886f6636"
       url: "https://pub.dev"
     source: hosted
-    version: "0.7.10"
+    version: "0.7.9"
   timing:
     dependency: transitive
     description: