Demote ComparableType errorprone warning

So that it can be promoted to an error by default.

Bug: 253827323
Test: m RUN_ERROR_PRONE=true javac-check
Change-Id: I952b13f86d0827d05e434120f193488730782d05
diff --git a/Android.bp b/Android.bp
index 6830bac..9394740 100644
--- a/Android.bp
+++ b/Android.bp
@@ -90,6 +90,11 @@
         "--add-exports jdk.javadoc/jdk.javadoc.internal.doclets.formats.html=ALL-UNNAMED",
         "--add-exports jdk.javadoc/jdk.javadoc.internal.tool=ALL-UNNAMED",
     ],
+    errorprone: {
+        javacflags: [
+            "-Xep:ComparableType:WARN",
+        ],
+    },
 }
 
 java_library_host {