Revert "Update project.path var to new location. (#3985)" (#4073)

This reverts commit 089c828.

Author: vkryachko

buildSrc/src/main/java/com/google/firebase/gradle/plugins/JavadocPlugin.java

@@ -188,8 +188,7 @@ private static void applyDoclava(Project project) {
                   .setValue(
                       ImmutableList.of(
                           ImmutableList.of("book.path", "/docs/reference/_book.yaml"),
-                          ImmutableList.of(
-                              "project.path", "/docs/reference/android/_project.yaml")));
+                          ImmutableList.of("project.path", "/_project.yaml")));
 
               // root path assumed by docs
               options.addStringOption("toroot", "/docs/reference/android/");