Print with newline for ui_print.

Currently the ui_print command between the recovery and updater doesn't
append newline. Updater has to send an extra "ui_print" command without
any argument to get the line break. This looks unnecessary. And not all
the callers (including the ones in bootable/recovery) are following this
protocol when sending the ui_print command.

This CL simplifies the protocol to always print with a newline for
ui_print command. When updating from an old recovery with the new
updater, all the ui_print'd strings would appear in one line as a side
effect. But a) it would only affect the text-mode UI, which won't be
shown to users; b) log files won't be affected.

Bug: 32305035
Test: Apply an update with the new updater on top of an old and new
      recovery image respectively.
Change-Id: I305a0ffc6f180daf60919cf99d24d1495d68749b