Introduce AndroidEnvConfig.

[copybara-service]

Introduce AndroidEnvConfig.

This change introduces AndroidEnvConfig, the topmost dataclass to configure
AndroidEnv's instantiation. This aligns the interface used in the open-source
version with Google's internal version.

The main diffs are in loader.py and loader_test.py, with two significant
differences:

• No automagic param manipulation for connecting to existing emulators.
  • Instead of trying to guess what clients want, loader.load() now just
    interprets the given settings, including the Emulator's console port and
    adb port. This is less error-prone and much simpler to use.
• loader.load() now supports loading AndroidEnv with a FakeSimulator.
  • This is very convenient when setting up AndroidEnv in a more complex
    infrastructure since FakeSimulator boots up instantaneously.