Change default heap-target-utilization to 0.75

On high-memory devices we often use system properties to explicitly use
more relaxed heap-target-utilization. So changing the default value
to a more conservative one for low-memory devices.

Bug: 152837465
Test: art/test/testrunner/testrunner.py
Change-Id: I03b6af27fe246683b0282cf1ee136b287dee729d
(cherry picked from commit 963c27f6035db91e99c08bb52101c8acfc0af4d3)
1 file changed