Support building Android libs with Java 11 targetting Java 8

[codeanticode]

Processing-core library build with Java 11 results in the following error when running sketches from the PDE:

java.lang.NoSuchMethodError: No virtual method rewind()Ljava/nio/ByteBuffer; in class Ljava/nio/ByteBuffer; or its super classes (declaration of 'java.nio.ByteBuffer' appears in /system/framework/core-oj.jar)

Apparently this is caused by the library being built against the wrong JRE (see this) although the top level gradle build file already sets source and target compatibility to 1.8.

[codeanticode]

Sounds like setting the source and target compatibility to 1.8 is not enough because of the following:

https://stackoverflow.com/questions/43102787/what-is-the-release-flag-in-the-java-9-compiler

And the release compiler flag can be set in the gradle build file as well:

https://docs.gradle.org/current/userguide/building_java_projects.html#sec:java_cross_compilation

This should do it, but then the packages sun.security.pkcs.* used in the JarSigner are not recognized by the compile (#627)... so switching to Java 8 to build the library for the time being.

[codeanticode]

Added option to top-level build file in de89d88, will leave commented out until full transition to Java 11 is completed.