Only compile hot methods

Instead of compiling all startup + hot methods, we now only compile
hot methods. However, the current logic still marks all startup
methods that have a counter greater than hot_method_sample_threshold_
as hot methods. Since hot-startup-method-samples is currently 1,
there is no current behavioral change.

The goal is to increase hot_method_sample_threshold_ in a follow up
CL.

Renamed startup-method-samples to hot-startup-method-samples to
better reflect the new behavior of the option.

Test: test-art-host
Bug: 36457259

Change-Id: I820bdcd4426769d76131b08a8b41f3b7eebfbc23