(#211) Fix Usage Screen retry when returning from network error

[ryanw-mobile]

In UsageViewModel, we forgot to specify requestedScreenType = UsageScreenType.Chart, when we want to show the chart. Since this was a default value we were not aware of it until ErrorScreen alters this value.

A quick fix to add back this value.

Also refactored a bit the ViewModel and other classes.

[codacy-production[bot]]

Coverage summary from Codacy

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Coverage variation	Diff coverage
:white_check_mark: -0.03% (target: -1.00%)	:white_check_mark: 25.49%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (3420024f7805abc3d7162d4fa2ef3172c61b1c84) | 1825 | 974 | 53.37% | | | Head commit (cb512b5c136e58fe7f226a231f28c3f5aea791de) | 1826 (+1) | 974 (+0) | 53.34% (**-0.03%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#226) | 51 | 13 | **25.49%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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