fix: add namespace (AGP8)

[MrCyjaneK]

AGP8 requires build.gradle to add namespace.

 > Could not create an instance of type com.android.build.api.variant.impl.LibraryVariantBuilderImpl.
   > Namespace not specified. Specify a namespace in the module's build file: /home/runner/.pub-cache/hosted/pub.dev/ledger_usb-1.0.0/android/build.gradle. See https://d.android.com/r/tools/upgrade-assistant/set-namespace for information about setting the namespace.