A better way to accommodate custom adb calls

This came up in a discussion with @winterfroststrom

The `AdbProxy` was designed for the most straightforward use cases, like `adb.shell('getprop')` - simple call and return.

This does not work well if users want to have more control of the call.
E.g. process the live streaming of the stdout of the call subprocess.

We might want to consider providing a new adb interface for users who want to make long-running calls and have more control of the subprocess.