Puts the common sources in an explicit Gradle sub-project rather
than just referring to them from multiple sourceSets.

Pro: Project now loads cleanly in Studio with no duplicate source
root warnings and Studio can follow cross-refs between all
sub-projects.

Pro: Also tidied up the dependencies to the constants build.
Hopefully we can now tidy that build up further.

Con: Had to add a whole new set of stubs (platform-stub) for
Platform.java due to circular dependency... All of the code in
common/ expects to be compiled against the current platform's
Platform.java but all of the Platform.java's also expect to be
compiled against the common code.

Pro: Adding platform-stub allows us to remove the platformJar
stuff from the OpenJDK build. Turns out this was just a hack to
allow android-platform to build on OpenJDK so compiling against a
stub is cleaner.

Con: the common build's processTestResources task seems to try
and copy all the resources twice causing an error.  Added a hacky
workaround for now.

Major con: The AARs built by the Android build do not contain
classes from the common sources. I've tried multiple ways of
expressing the dependency and got nowhere so advice would be
appreciated.

Still various Javadoc issues remain but they existed before this PR.

When specified in a defaultConfig block it ends
up setting the module version too.....

Was using a no-longer-needed 3P plugin which doesn't
work correctly with the latest AGP.  Now is at least
publishing an AAR even though it doesn't contain
the common classes.

1. Remove the implementation dependency from :conscrypt-android
to :conscrypt-common as that ends up exposing conscrypt-common
as a dependency in the POM file.

2. Just copy the actual class files from the :conscrypt-common
output into the :conscrypt-android build directory before the
AAR is built.