Convert MultiActivityTestBase children to JUnit4

The follows the same pattern created for chrome activity test rules and test base,
where test rules and test bases share as much same implementation as possible in
the test common classes during the migration. For more on JUnit4 migration, please
check src/testing/android/docs/junit4.md

BUG=640116

Review-Url: https://codereview.chromium.org/2847933002
Cr-Commit-Position: refs/heads/master@{#468521}
6 files changed