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Introduces the @Batch annotation, which causes the test runner to run
tests without a separate instrumentation run for each test (in other
words, no Activity/process restart).

Lowers the per-test overhead from around 10 seconds on my device to <5ms
for all tests with this new annotation. (Bots from ~2s to <5ms)

The BATCH.UNIT_TESTS batch is for unit tests that don't require any
globally shared setup/teardown and don't change global state.

The Batch.PER_CLASS batch is for integration tests that don't require
a restart between each test, but do require a restart before/after the
suite runs. It is equivalent to @Batch('unique_name').

In the future, batches may be added at other layers for suites that
agree on known starting conditions, and reset to those starting
conditions.

This change also removes some dead handling of junit3 tests so that we
can re-interpret an array of tests as a collection of tests to run in a
single instrumentation call.

Some followup work will be required to fix reported duration, as each
test individually is reported as taking the duration the batch of tests
took.

Bug: 989569
Change-Id: I2fee81c33363789ca63174432a3b90b4fb472097
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2199706
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771630}