Keep track of primary dexopt

In commit ce543973, the results of both secondary dexopt and
primary dexopt were clubbed. This CL partially reverts the change
to not aggregate the two dexopt in single stat. A TODO has been
added to store stats for secondary dexopt.

Bug: 38413085
Test: build successful
Change-Id: I7ab437c694d5eb7df5f03706ace26380f513489e