Stop using a regex for setupAppDir.

This was hard to read and understand. Instead, fall back to explicit
string operations with more comments on what we're doing and what we're
allowing.

This also fixes an issue where apps were asking us to create dirs on
their behalf that our more than 2 levels deep, eg
com.foo/files/downloads ; I thought such paths weren't allowed, but
apparently they are (and there's no good reason for us to not set them
up correctly).

Bug: 149407572
Test: launch opera
Change-Id: I7c64831032b66e90960b96e41ee42c7d616a759c