remount: Increase scratch size from 50% to 85% of available data

adb remount clearly indicates that the user will use
a scratch partition to modify RO content. In this CL we're
trying to be more aggressive and take more space from
data to scratch to improve the developers experience.

Bug: 228945443
Test: launch cvd with updated value and check it reflects
Change-Id: Ied9cee6e98d3b2dd594babcf213071a80bd3cf14
Signed-off-by: Dmitrii Merkurev <dimorinny@google.com>