Android: Make emulator process known to mill

[vaslabs]

API changes

./mill app.startAndroidEmulator
./mill app.stopAndroidEmulator

to

./mill app.androidEmulator
./mill clean app.androidEmulator

Potential alternatives androidEmulatorRun or runAndroidEmulator or keep start

Implementation example

See the MillBackgroundWrapper example here

(Maybe) Related to #6522

[souvlakias]

Currently the stopAndroidEmulator uses adb ... kill which tells the emulator to stop, and allowing it to save its state and shutdown normally.

If we use MillBackgroundWrapper as is, that would stop the emulator by just sending a kill signal which is not ideal for this case.
(Interestingly, when manually killing the pid of the emulator the Saving state... popup sometimes does appear but is not consistent)

So we can either:

• Modify the shutdown hook of the MillBackgroundWrapper on runtime (not sure if this is doable)
• Reimplement MillBackgroundWrapper in androidlib but with our custom shutdown hook
• Maybe something else :D?

[lefou]

Shouldn't this be implemented as a worker instead?