Adds initializer for Rive for use at app startup

[mjohnsullivan]

Creates an Initializer for Rive that can used to initialize the runtime at startup either through declaring an InitializationProvider in the manifest, or calling AppInitializer directly in the code:

<provider
     android:name="androidx.startup.InitializationProvider"
     android:authorities="${applicationId}.androidx-startup"
     android:exported="false"
     tools:node="merge">
     <meta-data android:name="app.rive.runtime.kotlin.RiveInitializer"
         android:value="androidx.startup" />
 </provider>

AppInitializer.getInstance(applicationContext)
  .initializeComponent(RiveInitializer::class.java)

This follow Android's current guidance for initializing components at startup: https://developer.android.com/topic/libraries/app-startup

[mjohnsullivan]

Documented in RiveInitializer.kt

[mjtalbot]

as before, you mentioned you may have another idea here. so i will leave this sitting fttb. lgtm though

[mjohnsullivan]

Moved initializer to module and appears to be working; ready for merging.