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/remote_track_publication.dart

@@ -7,7 +7,7 @@ import 'track_publication.dart';
 /// Represents a track publication from a RemoteParticipant. Provides methods to
 /// control if we should subscribe to the track, and its quality (for video).
 class RemoteTrackPublication extends TrackPublication {
-  RemoteParticipant _participant;
+  final RemoteParticipant _participant;
   bool _unsubscribed = false;
   bool _disabled = false;
   VideoQuality _videoQuality = VideoQuality.HIGH;
@@ -26,6 +26,7 @@ class RemoteTrackPublication extends TrackPublication {
     _sendUpdateTrackSettings();
   }
 
+  @override
   bool get subscribed {
     if (_unsubscribed) {
       return false;
@@ -41,6 +42,7 @@ class RemoteTrackPublication extends TrackPublication {
 
   /// for internal use
   /// {@nodoc}
+  @override
   set muted(bool val) {
     if (val == muted) {
       return;
@@ -59,13 +61,12 @@ class RemoteTrackPublication extends TrackPublication {
     _participant.muteChanged();
   }
 
-  RemoteTrackPublication(TrackInfo info, this._participant, [Track? track])
-      : super.fromInfo(info) {
+  RemoteTrackPublication(TrackInfo info, this._participant, [Track? track]) : super.fromInfo(info) {
     this.track = track;
   }
 
-  _sendUpdateTrackSettings() {
-    var settings = new UpdateTrackSettings(
+  void _sendUpdateTrackSettings() {
+    final settings = UpdateTrackSettings(
       trackSids: [sid],
       disabled: _disabled,
     );