Update INSTALL-ANDROID.md

[millerf]

I have been struggling to make Android work because installation was not very clear...

As it is written at the moment, I did put the ext{} object at the root of the file.

I would get this error which can be hard to understand

> Cannot get property 'compileSdkVersion' on extra properties extension as it does not exist

This change should make this clearer, by referencing allprojects{}