DialogHandler assumes it was started by DeckPicker, may crash otherwise

###### Reproduction Steps

1. Open AnkiDroid via a Reviewer-first pathway?
2. request file import?
3.

###### Expected Result

Import dialog is shown

###### Actual Result

ClassCastException because import dialog is spawned from Reviewer somehow, instead of DeckPicker?
```
java.lang.ClassCastException: com.ichi2.anki.Reviewer cannot be cast to com.ichi2.anki.DeckPicker
at com.ichi2.anki.dialogs.DialogHandler.handleMessage(DialogHandler.java:82)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6501)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
```