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

See go/jetpack-test-android-migration

Test: atest TestablesTests NetworkStackTests FrameworksServicesTests SystemUITests FrameworksUiServicesTests ExtServicesUnitTests
Exempt-From-Owner-Approval: already reviewed
Change-Id: I083fcdaa71b503535aa2b3c257740f3ecf055373
Merged-In: Ib04d80954bd8536914d88c66b28e6632e60a6245