Migrate frameworks/base/tests/testables and its users to androidx.test

See go/jetpack-test-android-migration

This is the internal version of AOSP change aosp/908373

Test: m -j TestablesTests NetworkStackTests FrameworksServicesTests SystemUITests FrameworksUiServicesTests ExtServicesUnitTests WmTests FrameworksCoreTests

Exempt-From-Owner-Approval: automated package name refactoring; already reviewed on AOSP
Change-Id: Ib04d80954bd8536914d88c66b28e6632e60a6245