Fixes for signed build

Fragments can only have a default constructor, switch to newInstance
paradigm.

It's possible that the current tone is a RingPack tone not in our list
of Packs, so avoid NullPointerException if that's the case.

When disabling, why not delete all instances of ourselves in the
MediaStore that we can find?
Safer RingPack parsing

We should never crash now, simply fail to add it and either alert the
user or log it.
Add release instructions

Gradle knows how to build a signed APK now.