[Android] replace system ClassLoader with fully working ClassLoader.

[atsushieno]

(redoing #1038, sorry for the noise!)

This fixes one of the issues exposed as https://forum.juce.com/t/env-findclass-does-not-work-inside-juce-app-very-strange/46886 . A fully detailed explanation is available at https://atsushieno.github.io/2022/03/16/juce-android-threads.html

In the current JUCE JNI invocation foundation (JNIClassBase etc.), the Android system ClassLoader was used. It turned out that it is different from the actual ClassLoader that we see in Dalvik bytecode VM, and the current one fails to load non-system and non-pre-registered Java classes. It can be reproduced by adding some code before call to Java.initialiseJUCE() in JuceApp:

    this.getClass().getClassLoader().loadClass("com.rmsl.juce.Java");
    ClassLoader.getSystemClassLoader().loadClass("com.rmsl.juce.Java");
    com.rmsl.juce.Java.initialiseJUCE(context.getApplicationContext());

A fully working ClassLoader can be retrieved from com.rmsl.juce.Java Class using getClassLoader(). Use it instead of system ClassLoader.

This needs additional change on where to call initialiseAllClasses() (move from JNI_OnLoad() to Thread::initialiseJUCE() as we need a Dalvik bytecode VM.