Regenerate the background text with pngcrush

After process the text images with pngquant, I encountered a
image-loading error when doing graphic tests on angler. The command
"pngcrush -c 0 $imagename" resolves this error and gives almost the
same compression rate. So regenerate all the text images with "pngcrush
-c 0"

Bug: 30415666
Test: Run graphics test on angler and the background text looks good.
Change-Id: I5d989c1230e826407287265fc559349a7bd29303