old p4a did not work with new buildozer. kivy master crashes.
kivy latest release has runtime issues (orientation was landscape).
these versions seem to work.
also updated dockerfile to more closely match p4a master.
so that we don't need the extra permission.
also because phones these days have enough internal storage for
the headers; and maybe it's better even for security reasons to
store it there.
no upgrade path is provided for the headers stored on external storage,
we will litter the filesystem and leave them there. they will be
downloaded again into internal storage.
