Glimpse: Rework view activity current media tracking

* Due to submitList being async, we don't get the most recent media object when using getItemAtPosition.
* We now instead rely on the adapter pushing to us the currenly shown media.
* While at it, make sure buttons are shown only when supported.
* Also let us use the "use as" feature for all types of media and delete forever only for MediaStore medias

Change-Id: I2e4bac05eed976971dc8c6e9a13335335b2cac0f