IssueTracker

It'd be great to include a scoping mechanism for ComponentInitializer's. This would allow component initializers to either run before the application class is created (any app launch, including background), or before the first activity is launched (user-initiated app launches).

One way around this, preserving the `ComponentInitializer` interface and behaviour of the `initializeComponent` method would be to include another meta-data value denoting scope, and rename the current one for clarity. For example:

```xml
<!-- Application Startup -->
<meta-data
    android:name="androidx.startup.second_library.DependentComponentInitializer"
    android:value="@string/androidx_app_startup" />

<!-- Activity Startup -->
<meta-data
    android:name="androidx.startup.second_library.DependentComponentInitializer"
    android:value="@string/androidx_activity_startup" />
```

`AppInitializer.getInstance(..).initializeComponent(..)` could work regardless of scope, and the discovery mechanism could be changed slightly to accommodate the new meta-data value.