Set Java version for java_api_library module

Currently, stubs are compiled with Java version 1.8.
To align with this, set the java version passed to javac invocation as
1.8 for java_api_library module. Helper function is used to sync the
java version of droiddoc module and java_api_library module.

Test: m
Change-Id: I103b94aed9747a2f2dd65d13de0ae9c0f33db5c0