[flutter_webrtc] Fix analyze issue

[JSUYA]

Resolve following issues:

error • example/flutter_webrtc_example/lib/main.dart:31:59 • There's no constant named 'Default' in 'AndroidNotificationImportance'. Try correcting the name to the name of an existing constant, or
        defining a constant named 'Default'. • undefined_enum_constant
error • example/flutter_webrtc_example/lib/src/get_display_media_sample.dart:64:41 • Invalid constant value. • invalid_constant error • example/flutter_webrtc_example/lib/src/get_display_media_sample.dart:64:71 • There's no constant named 'Default' in 'AndroidNotificationImportance'. Try correcting the name to the name of an
        existing constant, or defining a constant named 'Default'. • undefined_enum_constant
 info • example/flutter_webrtc_demo/lib/src/utils/screen_select_dialog.dart:279:22 • 'ButtonBar' is deprecated and shouldn't be used. Use OverflowBar instead. This feature was deprecated after
        v3.21.0-10.0.pre. Try replacing the use of the deprecated member with the replacement. • deprecated_member_use
 info • example/flutter_webrtc_demo/lib/src/utils/screen_select_dialog.dart:279:22 • 'ButtonBar.new' is deprecated and shouldn't be used. Use OverflowBar instead. This feature was deprecated after
        v3.21.0-10.0.pre. Try replacing the use of the deprecated member with the replacement. • deprecated_member_use
 info • example/flutter_webrtc_demo/lib/src/widgets/screen_select_dialog.dart:279:22 • 'ButtonBar' is deprecated and shouldn't be used. Use OverflowBar instead. This feature was deprecated after
        v3.21.0-10.0.pre. Try replacing the use of the deprecated member with the replacement. • deprecated_member_use
 info • example/flutter_webrtc_demo/lib/src/widgets/screen_select_dialog.dart:279:22 • 'ButtonBar.new' is deprecated and shouldn't be used. Use OverflowBar instead. This feature was deprecated after
        v3.21.0-10.0.pre. Try replacing the use of the deprecated member with the replacement. • deprecated_member_use
 info • example/flutter_webrtc_example/lib/src/loopback_sample_unified_tracks.dart:847:24 • 'ButtonBar' is deprecated and shouldn't be used. Use OverflowBar instead. This feature was deprecated after
        v3.21.0-10.0.pre. Try replacing the use of the deprecated member with the replacement. • deprecated_member_use
 info • example/flutter_webrtc_example/lib/src/loopback_sample_unified_tracks.dart:847:24 • 'ButtonBar.new' is deprecated and shouldn't be used. Use OverflowBar instead. This feature was deprecated
        after v3.21.0-10.0.pre. Try replacing the use of the deprecated member with the replacement. • deprecated_member_use
 info • example/flutter_webrtc_example/lib/src/widgets/screen_select_dialog.dart:279:22 • 'ButtonBar' is deprecated and shouldn't be used. Use OverflowBar instead. This feature was deprecated after
        v3.21.0-10.0.pre. Try replacing the use of the deprecated member with the replacement. • deprecated_member_use
 info • example/flutter_webrtc_example/lib/src/widgets/screen_select_dialog.dart:279:22 • 'ButtonBar.new' is deprecated and shouldn't be used. Use OverflowBar instead. This feature was deprecated after
        v3.21.0-10.0.pre. Try replacing the use of the deprecated member with the replacement. • deprecated_member_use