Allow tuning of heaptargetutilization

Remove a pair of explicit sets of this value.  This allows ART to tune
this parameter to improve GC responsiveness and memory usage.  Tuning
this parameter is a key area of work for OEM-requested high-memory
device configurations.

Bug: 145823510
Test: boot and run with various configurations of the flag.
Change-Id: I19680ff5fa1ebf9dfd4a3f71533d03510f4da414