Update system.prop

Remove persist.sys.use_dithering
This commit automatically enables dithering if bitsPerPixel <= 16
https://github.com/androidarmv6/android_frameworks_native/commit/05f952e8cf2972995f61754dbe4cefb415725881

Update dalvik.vm.heaptargetutilization=0.25
Reduces after boot memory footprint by about 5-8MB

Change-Id: I636c33bc0710fbdff14e832d04b00b752618a1a5