[Example] fix android & iOS not building

[chipweinberger]

The example app needed some tender loving care.

It was not building on latest flutter & android due to Kotlin: Unsupported version of com.android.tools.build:gradle plugin: version 4.2.2 or higher should be used with kotlin-android plugin

deleting the android folder and re-running flutter create fixed that issue^, and also made a bunch of other small upgrades to current flutter best practices.

• [Example] iOS: re-run flutter create to update to current recommended project setup
• [Example] android: re-run flutter create to update to current recommended project setup
• [Example] update pubspec.yaml to latest versions
• [Example] center buttons, fix warnings

I've tested the app on Android 13, and iOS 17.2.1 and Flutter 3.16.8

About me, I am the maintainer of these packages, so I am pretty familiar with flutter plugins

• flutter_blue_plus
• flutter_pcm_sound
• dart_melty_soundfont
• flutter_quick_video_encoder