* Improved notification management.
* Implemented downloading from the Google Play Store.

staging.termux-mirror.ml will be down and shouldn't be used.

* Changed the hardcoded cache directory to the android alternative (pointing to /data/user/0 which is a symlink to /data/data).
* Implemented the transparent activity mechanism for downloading multiple apps through the Play Store (which just works as if the intent was added to the back-stack)
* Now we check deliberately check for source (external for ext. repo and play-store for the Play Store).
* Fixed a few bugs.

…re versions

On Android 11 we need MANAGE_EXTERNAL_STORAGE to access files without
restrictions on shared storage.

Android 10 we have to opt-out from scoped storage and use WRITE_EXTERNAL_STORAGE.

…GE granted