Add a default dispatcher to the KeyedQueryPagingSource backed QueryPagingSource function

[veyndan]

This is expected, as per the documentation:

sqldelight/docs/android_sqlite/android_paging.md

Line 95 in e4b1064

By default, queries are performed on `Dispatchers.IO` if no dispatcher is specified. Consumers expecting to use RxJava's `Scheduler` to perform queries should use the [`Scheduler.asCoroutineDispatcher`](https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-rx2/kotlinx.coroutines.rx2/io.reactivex.-scheduler/as-coroutine-dispatcher.html) extension function.