* Do not ask for WRITE_EXTERNAL_STORAGE in Android 11+ (writing to "Downloads" is [allowed by default since 11](https://stackoverflow.com/questions/70248631/starting-from-android11-do-i-need-to-comply-to-androids-saf-just-to-even-creat)).
* Do not check for permissions before receiving a file. Try and let it error out, instead of silently doing nothing.
* Fix not offering to open content:// urls (we checked the URLs to be file:// urls to then convert them to content://).
* Better permission explanation text.
Removes conditional code for older Android versions (IceCreamSandwitch, JellyBean, KitKat, Lollipop) since we no longer support Android older than Lollipop (5.0).
This doesn't remove KitKat support in the SFTP plugin since it's a bit more convoluted. It will be done in a separate MR.
Motivation: We can't publish to the Play Store with a targetSdkVersion < 31
The new targetSdk required two changes:
* Specifying when activities, services and receivers should be exported or
not to other apps. For the ones I wasn't sure of what we want, I made
them exported to not break something.
* Specifying when a PendingIntent should be mutable or immutable. I made
them all mutable for now.
