bpo-29240: PyUnicode_DecodeLocale() uses UTF-8 on Android (#5272)

PyUnicode_DecodeLocaleAndSize(), PyUnicode_DecodeLocale() and
PyUnicode_EncodeLocale() now use always use the UTF-8 encoding on
Android, instead of the current locale encoding.

On Android API 19, mbstowcs() and wcstombs() are broken and cannot be
used.