Simpler Qt for Android Java bindings

The current Qt for Android Java bindings have few issues that might affect the stability and performance of Qt apps

1. The public bindings are too complex and big with no real user to users.​
2. Some parts of the bindings should be internal, yet we have them public (Loader parts).​
3. The architecture and relation is convoluted between the Loader, Delegate and the facing Activity class.​
4. Heavy use of reflection which is a bit slow and hard to maintain.​​

As part of this work, we want to simplify the architecture, improve the responsibilities of each part, i.e. the public bindings shouldn't do much of the internal logic, and complex relations should be broken down to simpler ones.​

As part of this work, it's expected to get rid of the multiple code duplications in the Java bindings and internal codes.