ActivityScenarioRule launches an EmptyActivity in between each test

[ChrisCraik]

This adds several hundred ms to each test using the Rule.

The separate launch should be confined to only the affected API levels:

android-test/core/java/androidx/test/core/app/InstrumentationActivityInvoker.java

Line 469 in ef59895

public void finishActivity(Activity activity) {

Additionally, consider disabling animations with overridePendingTransitions(0,0) for EmptyActivity and similar, since most tests run with window animations enabled (and they're difficult to disable in Studio: https://issuetracker.google.com/issues/138120842)