releasetools: Fix an issue when using APEX container key mapping.

The keys_info in the touched code is a tuple, which is immutable.

Bug: 123716522
Test: Run sign_target_files_apks.py with '-e foo.apex=bar' that replaces
      the APEX container key.

Change-Id: I4e57e46c93a56b7f6646764d021ebb42c19bf7f5
diff --git a/tools/releasetools/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index a9d2218..f1f032d 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -193,7 +193,7 @@
     if apex not in keys_info:
       continue
     assert key, 'Presigned APEX container for {} is not allowed'.format(apex)
-    keys_info[apex][1] = key_map.get(key, key)
+    keys_info[apex] = (keys_info[apex][0], key_map.get(key, key))
 
   return keys_info