Clean up ActivityTestRule inheritance

The pattern of:

Statement selfStatement = {
<before>
base.evaluate();
<after>
}
return super.apply(selfStatement, description);

is counter-intuitive in derived test rule classes, because |base| is not
the parent class's statement. super.apply causes the statements to be
chained, not extended like normal inheritance, so if there are ordering
dependencies in inherited test rules it's difficult to manage them and
reason about them.

Much simpler and less boilerplate to use ExternalResource's before/after.

Change-Id: I0ea1e9348d9ab06e1bf8e5dfabdebac6e948803f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5605668


Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312987}