Android: create an AndroidEmulatorWorker

[souvlakias]

Motivation

Have mill be aware of the emulator process

Api Changes

Instead of using {start,stop}AndroidEmulator tasks to start and stop the emulator:
Have one task: androidEmulator that calls the AndroidEmulatorWorker worker which handles the adb kill logic when stopped. e.g. with ./mill clean app.androidEmulator

Closes #6528