fixes some out-of-date code and warnings

This merge updates some code to support API 29 and fixes some inspection warnings.

Important note: For reasons not yet clear to me, the inspector was throwing some "Code maturity" warnings about file- and photo-saving when I started this work. I understood this to be the result of the update to Android Q (cf. "The Death of External Storage: The End of the Saga(?)"). However, those warnings are now gone, and the only change I made was to update something through "Check for updates" in Android Studio. This merge trusts the current status of the inspector, which is content with our existing approach to saving files and photos.

Closes #32 (closed).