Fix a race condition during Context init/teardown.

This change makes Context initialization wait properly for all threads to be
available before leaving initContext(). This prevents a case where ~Context()
gets called before all created threads have started/finished their work. This
was triggered/discovered by running some very short-lived tests in CTS.

BUG=4064203

Change-Id: I1dd6f43fa7942c53881008e857df864e2395202e