updater: Fix the line breaks in ui_print commands.

When processing ui_print commands in the updater, it misses a line break
when printing to the recovery log.

Also clean up uiPrintf() and UIPrintFn() with std::string's.

Change-Id: Ie5dbbfbc40b024929887d3c3ccd3a334249a8c9d
1 file changed