Move error_prone config to external/error_prone

The error_prone config will likely need to be updated at the same
time as the error_prone prebuilts, so move the config into the same
project.  This requires jumping through some hoops because
external/error_prone may not be present in all manifests.

Bug: 64489631
Test: m -j RUN_ERROR_PRONE=true
Change-Id: I2c59df1148134ffedac0e00d32ac7082a9fb5330

Android.bp

@@ -222,7 +222,7 @@ bootstrap_go_package {
     ],
     srcs: [
         "java/config/config.go",
-        "java/config/errorprone.go",
+        "java/config/error_prone.go",
         "java/config/makevars.go",
     ],
 }