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
diff --git a/java/droiddoc.go b/java/droiddoc.go
index 8a291ad..aa55f37 100644
--- a/java/droiddoc.go
+++ b/java/droiddoc.go
@@ -598,7 +598,7 @@
 	// Droiddoc always gets "-source 1.8" because it doesn't support 1.9 sources.  For modules with 1.9
 	// sources, droiddoc will get sources produced by metalava which will have already stripped out the
 	// 1.9 language features.
-	cmd.FlagWithArg("-source ", "1.8").
+	cmd.FlagWithArg("-source ", getStubsJavaVersion().String()).
 		Flag("-J-Xmx1600m").
 		Flag("-J-XX:-OmitStackTraceInFastThrow").
 		Flag("-XDignore.symbol.file").