Increase test runner thread count for target.

Experiment.

There was no point in increasing the thread count before since
the buildbots were fully bottlenecked by host java compilation.
This has been eliminated by moving the compilation to soong.

Try to slightly increase the thread count to see if we get any
boildbot speedup or if we start having timeout/flake issues.

Test: art/test.py --target -r --optimizing
Change-Id: Iaa705f39e57f09131a6ed65e23d1ca995d3e4fd4
diff --git a/test/testrunner/testrunner.py b/test/testrunner/testrunner.py
index 82ad0c8..935ce0c 100755
--- a/test/testrunner/testrunner.py
+++ b/test/testrunner/testrunner.py
@@ -319,8 +319,8 @@
   if 'target' in _user_input_variants['target']:
     device_name = get_device_name()
     if n_thread == 0:
-      # Use only half of the cores since fully loading the device tends to lead to timeouts.
-      n_thread = get_target_cpu_count() // 2
+      # Use only part of the cores since fully loading the device tends to lead to timeouts.
+      n_thread = max(1, int(get_target_cpu_count() * 0.75))
       if device_name == 'fugu':
         n_thread = 1
   else: