init: Change extra_free_kbytes.sh to add margin relative to the default watermark level

extra_free_kbytes.sh accepts a parameter representing the number of KB
to add to low and high watermarks. It adds this margin to the current
watermark levels, however this is not how /proc/sys/vm/extra_free_kbytes
knob that it replaces used to work. The old knob would add the margin
relative to the original and not the current level of the watermarks.
Change extra_free_kbytes.sh to add the specified margin to the original
watermark levels to act as correct replacement of the old knob.

Bug: 242837506
Fixes: 642048d9 ("init: Add extra_free_kbytes.sh script to adjust watermark_scale_factor")
Test: repeatedly run 'setprop sys.sysctl.extra_free_kbytes 30375'
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I8af603ba00bd4fb8182b80b3c0969fa96cdd7311