Add Instructions on Installing Java from Oracle Website

[walidabualafia]

I was following steps on installing the correct version of Java, and found that this document needed completion.

Added instructions on installing Java from website in android/configure_java.md. Previously, Method #3: install from website was marked "TODO".

Simply, the instructions follow these steps:

1. Download the Java 8 JDK package from Oracle Downloads
2. Launch the package and install Java on macOS
3. Make $JAVA_HOME env variable point to the JDK install by adding it to shell startup file

I also added a link to Java Docs on installing Java to macOS, if users still have trouble.