Don't try to remove Cargo.lock if it doesn't exist.

Change-Id: Ibfad6d97988edb6305d7a237d038412ec5a6100f
diff --git a/scripts/cargo2android.py b/scripts/cargo2android.py
index ddb7e86..dd6081f 100755
--- a/scripts/cargo2android.py
+++ b/scripts/cargo2android.py
@@ -1370,7 +1370,8 @@
     os.environ['PATH'] = saved_path
     if not self.dry_run:
       if not had_cargo_lock:  # restore to no Cargo.lock state
-        os.remove(cargo_lock)
+        if os.path.exists(cargo_lock):
+          os.remove(cargo_lock)
       elif not self.args.use_cargo_lock:  # restore saved Cargo.lock
         os.rename(cargo_lock_saved, cargo_lock)
     return self