Apply Flutter best practices [Non-Breaking] (#2)

* Add flutter_lints package

* Use key in widget constructors

lint: use_key_in_widget_constructors

* Unnecessary new keyword

lint: unnecessary_new

* Prefer const with constant constructors

* Exclude protobuf files from analyzer

* Private field could be final

* Annotate overridden members

* Use initializing formals when possible

* Don't access members with `this` unless avoiding shadowing

* Prefer is! operator

* Use isEmpty instead of length

* Use collection literals when possible

* Revert comment changes

* Cleaner null testing

* Misc fixes

* Avoid using `forEach` with a function literal

* Prefer `final` over `var` where applicable

* Enforce stricter type-checking

* Ignore VS Code files

* Flutter format

* Prefer using lowerCamelCase for constant names

* flutter format -l 100

* Disable `avoid_print` for examples

* Slight improvements to example

Should solve lint error: no_logic_in_create_state

lib/src/track/audio_track.dart

@@ -13,14 +13,14 @@ class AudioTrack extends Track {
 
   /// Start playing audio track. On web platform, create an audio element and
   /// start playback
-  start() {
-    if (!(this is LocalAudioTrack)) {
+  void start() {
+    if (this is! LocalAudioTrack) {
       audio.startAudio(getCid(), mediaTrack);
     }
   }
 
   @override
-  stop() {
+  void stop() {
     mediaStream?.dispose();
     mediaStream = null;
     audio.stopAudio(getCid());