Remove some attributes defined outside of __init__()

[Julian-O]

Attributes of a class instance should, where possible, be defined in the __init__() method, for clarity (especially about lifetimes). Pylint has a warning about this, but I can't see one from Flake8.

TargetAndroid has an attribute _serials which is defined (sometimes) outside of __init()__, and its value is checked with hasattr(). Changed it to act like a normal instance attribute, defined in __init__().

Buildozer() goes even further. It monkey-patches two attributes, _build_prepared and _build_done onto its associated target. These attributes have the same lifetime as the Buildozer instance. I removed the monkey-patching, and made them normal instance attributes of Buildozer, defined in __init__().

[misl6]

LGTM. Thank you!