androidenv: bump emulator version to latest stable - as per Linux

Move to a known version of the emulator.  Each OS has a distinct
version... pick the one on Linux.  A better solution would be to let
the user of `emulateApp` overload what the emulator version shall be
(and maybe it is already possible and I do not see it) - without need
to reproduce large portions of `default.nix`.

Using the previous emulator showed the following warning:
```
Your emulator is out of date, please update by launching Android Studio:
```

I am not aware of any reasons for not wanting the latest emulator (as
I expect it shall be compatible usually with more system images - not
less), so bump its default version.

Emulator release notes:
https://developer.android.com/studio/releases/emulator