Build IntelliJ project with full debug info

By default[1], javac only includes line number and source file
information in compiled outputs. Notably, this doesn't include local
variable debugging information, which means debugging tests in IntelliJ
won't be able to show local vars. Pass the necessary javac flag.

[1] http://docs.oracle.com/javase/7/docs/technotes/tools/windows/javac.html

Change-Id: I1d94ab3811ac590c9e533780d4e910cab6ff0506

.bazelproject

@@ -18,3 +18,6 @@ targets:
 java_language_level: 8
 
 workspace_type: java
+
+build_flags:
+  --javacopt=-g