[Java] Add Automatic-Module-Name to Manifest file

[danepitkin]

Add Automatic-Module-Name of com.google.flatbuffers to the Manifest file. Other google projects such as Guava have done this: https://github.com/google/guava/blob/3618043d084c07895b03bfa47b631b6cd54749a5/guava/pom.xml#L60-L69

This explicitly sets the module name so that if/when JPMS is supported in flatbuffers, it will not be seen as a breaking change for dependent Java projects that are modularized.