Fixes #43

As per https://developer.android.com/training/articles/security-config
Previous behaviour: only validated certificates in the system certificate authority store
New behaviour: validates against both system CAs and user-installed CAs

* [WIP] Use WorkManager (TODO: one-time vs periodic REPLACE)
* Use periodic work for synchronization (closes #7)
* WorkManager handles reboots

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

* add support for COLOR property (closes #31)
* Add tests, use new ical4android

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

* prevent crash if trying to load non existent calendar (closes #17)

* Minor changes

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

* give proper error message when a synced file is deleted (closes #19)

* extract strings to xml

* Also handle FileNotFoundException (for instance thrown by Google Drive when a file is not there anymore)

* Require context for taking permissions

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

Add starting framework for tests. Closes #10

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

Support Storage Access Framework

Co-authored-by: Ricki Hirner <hirner@bitfire.at>