Add system property for hot startup method threshold

The property is dalvik.vm.hot-startup-method-samples, setting this
controls how many startup samples are required before methods are
marked as hot in the profile.

Test: adb shell setprop dalvik.vm.hot-startup-method-samples 12
Test: adb shell setprop dalvik.vm.extra-opts -verbose:profiler
Test: adb logcat | grep Profile

Bug: 36457259

(cherry picked from commit 5eee004b)

Change-Id: Ibf8075aafc6e5bd2ba10385973b26faee3d807df