Fix #537: stop installing the deprecated tools package

Google no longer serves the `tools` package: it is gone from
repository2-3.xml, so `sdkmanager tools` fails with
"Failed to find package 'tools'" and takes the whole action down with it.

- Drop `tools` from the default value of the `packages` input, leaving
  `platform-tools`. The command line tools that replaced it are already
  installed by this action, and there is no bare `cmdline-tools` package
  path to install instead (only versioned ones, e.g. `cmdline-tools;20.0`).
- Skip `tools` with a warning when it is requested explicitly, so the
  many workflows that pass `packages: 'tools platform-tools'` keep working
  instead of failing.
- Update the README accordingly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

action.yml

@@ -21,7 +21,7 @@ inputs:
   packages:
     description: 'Additional packages to install'
     required: false
-    default: 'tools platform-tools'
+    default: 'platform-tools'
 
 runs:
   using: node24