Use only signed/unsigned numbers with ParseInt/ParseUint respectively

Test: build
Change-Id: If56b33c9c420237ff441779ba1dbebffd9dae8e3
diff --git a/updater/blockimg.cpp b/updater/blockimg.cpp
index 8388456..47849a1 100644
--- a/updater/blockimg.cpp
+++ b/updater/blockimg.cpp
@@ -109,7 +109,7 @@
     return false;
   }
 
-  if (!android::base::ParseInt(lines[0], last_command_index)) {
+  if (!android::base::ParseUint(lines[0], last_command_index)) {
     LOG(ERROR) << "Failed to parse integer in: " << lines[0];
     return false;
   }