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

Change-Id: If8e37a160802a00fb44f0c243fb358df02104741