fix(datastore): properly handle multiple configures on Android

[tyllark]

Issue #, if available:
#5244
#5458

Description of changes:
Calling configure multiple times on Datastore results in exceptions and other unexpected behavior:

1. Only add a user agent once when configuring multiple times. This ensures we properly throw AmplifyAlreadyConfiguredException rather than User-Agent was already configured successfully so it can be handle upstream.
2. Added missing cleanup code from within onDetachedFromEngine otherwise events would be duplicated from the listeners that were not cleaned up.
3. Made nativeAuthPlugin a companion object since we cannot add a new plugin after calling configure, but we need to handle the case where onDetachedFromEngine sets this value to null and it is reassigned via onAttachedToEngine from a new instance of AmplifyDataStorePlugin.

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