Fix race condition in release pages.

There was a race condition where another thread could coalesce the
free page run before we acquired the lock. In that case
free_page_runs_.find will not find a run starting at fpr. Added a
condition to handle this case. Also added handling for free page
runs with begin with released pages but end with empty pages.

Bug: 16191993
Change-Id: Ib12fdac8c246eae29c36f6a6728eb11d85553bbb