Reverts "Bump minSdk to 19 for Android tests" (flutter/engine#47935)
Reverts flutter/engine#47686
Initiated by: zanderso
This change reverts the following previous change:
Original Description:
The latest Robolectric version, 4.11.x, removed the support for SDKs < 19 by following AndroidX strategy. Also following https://github.com/flutter/buildroot/pull/750, and removing obsolete SDK checking in code to pass android_lint checking after bumping minSdk to 19 for testing target.
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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