androidlib plugins should default to Android

The directories with .androidlib "extension" should by default be marked as compatible only with Android.

 

Repro:

• Make a folder named "myplugin.androidlib"
• Copy it to Unity project
• Select the plugin and look into Plugin Inspector window
  • Expected: Plugin is marked as compatible only with Android
  • Actual: Plugin is marked compatible with any platform

Fix:

In PluginImporter.cpp we already have checks for .java, .kt and others to be marked as compatible with Android. Add .androidlib there too.