Bidi-format joined strings.

When joining strings, we want to respect RTL formatting for the
individual elements being joined, and joined string as a whole.

For example...
In LTR: "<LTR-formatted string>, <RTL-formatted string>"
In RTL: "<RTL-formatted string>, <LTR-formatted string>"

Bug: 20988802
Change-Id: I984ff364cbd0b0378bc467213274080e9524b18b