Exit with an error when the Android toolchain is not installed and `--platform` is set to 'android'

[acostalima]

Steps

• Check whether ANDROID_HOME is set
• Check whether ANDROID_SDK_ROOT is set
• Look up the tools in the appropriate directories:
  • platform-tools/adb
  • emulator/emulator
  • tools/bin/sdkmanager
  • tools/bin/avdmanager
• We may also check whether adb and emulator are available in the PATH

See:

• https://developer.android.com/studio/command-line/variables