Investigate using emulators for flutter/packages Android tests

[stuartmorgan]

Currently we use FTL for Android tests in CI; my understanding is that this is because emulators weren't an option. With the work to support emulators in LUCI, we should investigate migrating our Android integration tests to LUCI with emulators, instead of FTL:

• FTL does not run in presubmit for non-team members. This means we have to remember to manually push to PRs to trigger tests, and if we don't things like Revert "[camera] Add ability to concurrently record and stream video" plugins#6796 happen.
• FTL results are not available to non-team members, making debugging issues that don't happen locally very difficult.

We could potentially keep some additional tests if we still want device coverage, but it shouldn't be our sole way of running integration tests.

/cc @reidbaker