Bug 836377: Optimize size of dialer app's PNG files

The optimized PNG files have been stripped from their metadata and
reencoded with minimal bit depth by running

  optipng --strip -o7 *.png

The optimized PNG files are around 30-50% of the size of the original
one's.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Merge pull request #7970 from tdz/bug-836377

Bug 836377: Optimize size of dialer app's PNG files