* refactor(emulator)!: remove support for legacy `android` binary
`emulator.list_images` now always uses the `avdmanager` binary.
* refactor(android_sdk)!: remove support for legacy `android` binary
`android_sdk.list_targets` now always uses the `avdmanager` binary.
* refactor(check_reqs)!: do not look for legacy `android` binary
* refactor: replace installation instructions involving `android` binary