Have cargo2android put warning not to modify Android.bp files

Test: Run cargo2android
Test: Run upgrader script
Change-Id: Idd9fc9b8ad7a7c4fb3da954178d0221c95132fd3
diff --git a/scripts/cargo2android.py b/scripts/cargo2android.py
index 7d3b66a..ff3b468 100755
--- a/scripts/cargo2android.py
+++ b/scripts/cargo2android.py
@@ -89,7 +89,9 @@
 }
 
 # Header added to all generated Android.bp files.
-ANDROID_BP_HEADER = '// This file is generated by cargo2android.py {args}.\n'
+ANDROID_BP_HEADER = (
+    '// This file is generated by cargo2android.py {args}.\n' +
+    '// Do not modify this file as changes will be overridden on upgrade.\n')
 
 CARGO_OUT = 'cargo.out'  # Name of file to keep cargo build -v output.