gen_jarjar supported multi-valued arguments via space-separated lists,
like "--apistubs foo bar". This is unusual, as "bar" would generally be
interpreted as a positional argument instead.

Change the usage to "--apistubs foo --apistubs bar", which is more
conventional. Also remove the "--jars" switch to have the jars be
positional arguments instead, and change the "--unsupportedapi"
arguments to be a column-separated list (like java classpath), instead
of space-separated. For --unsupportedapi, this allows providing a list
of files via minimal modifications in the genrule usage.

Test: m
Change-Id: Id67ad16de2ea6682fee402540f464ed3b5b0cca9