Progress shown in snapcraft status output is not accurate

[Przemysław Suliga]

There are two parts here. First one likely won't need any action on the snapcraft side.

1. A Store bug where the current-percentage returned in the channelmap endpoint is not correctly split by architecture, i.e. current-percentage for a release represents current % of installs of a revision in a channel out of all installs in that channel (regardless of architecture), but it should be current % of installs in a channel for the architecture the revision is released for. This is going to be fixed on the store side very likely and it should not require any snapcraft changes. The end result should be that if snapcraft status reports progress of 10% -> 45% for armhf revision 245 released to stable, it should mean that the target is 45% of armhf installs in stable (this is correct currently) and 10% is 10% of all armhf stable installs and not 10% of all installs of any arch in stable.

2. When snapcraft reports a progressive release in `status` it currently calculates the current percentage of the non-progressive active release to be 100 minus $current-percentage-of-the-progressive-counterpart. This is not correct. In reality there will be active installs of revisions different than the current progressive one and current non-progressive one. After the Store fix for https://bugs.launchpad.net/snapcraft/+bug/1903046 is deployed, snapcraft can simply use the current-percentage of the non-progressive release instead (because current-percentage will be returned for all releases in the channelmap endpoint response).