[android] Fix main window not showing document in 5.2

This fixes BUG:488606 in 5.2 by flickering the main window after loading a document. In 5.3, this issue is fixed (somehow) by the existence of the text properties docker because it uses QML, so this workaround only needs to be applied to 5.2.

Test Plan

• Open or create a document on certain Android devices.
  • Previous behavior: the main window keeps showing a ghost image of the startup screen until the menu bar is fiddled with or Krita is sent to the background once.
  • New behavior: the document shows up properly without extra fiddling needed.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.