Ktor: switch engine from CIO to OkHttp for Android

[ryanw-mobile]

This is because to use Android Studio's network inspector, the easiest way is to change CIO to OkHttp without any extra configuration.

For practical consideration, we have no reason to stick to CIO

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.00% (target: -1.00%)	:white_check_mark: ∅

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (a3a078c9e35e1cae32b2ea383dd227a6d67be271) | 1705 | 780 | 45.75% | | | Head commit (22e95b78cc9d927132119fced0edf4d3a9d2d0a0) | 1705 (+0) | 780 (+0) | 45.75% (**+0.00%**) | **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 (#196) | 0 | 0 | **∅ (not applicable)** | **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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