Fix using aidl files from filegroups

Compute sources including from filegroup and genrule dependencies
before determining if any sources will cause flags to be added.

Test: gen_test.go
Change-Id: I0434b003bbda07a58bb2ce1a0a72997918c8fae2
diff --git a/Android.bp b/Android.bp
index a69c1ea..b57b0cc 100644
--- a/Android.bp
+++ b/Android.bp
@@ -158,6 +158,7 @@
     ],
     testSrcs: [
         "cc/cc_test.go",
+        "cc/gen_test.go",
         "cc/library_test.go",
         "cc/test_data_test.go",
     ],