bugfix: SessionTrack: strict ordering of queing a work

Holds a mutex in changing a flag and notifying.
wait_for() can takes 2 seconds when it receives cond_var
notification before flag setting.

Bug: 385027407
Test: l3 stress ranging
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:9cf4b74885ba78251796875422e57830645dc9a9)
Merged-In: I197037b95584e74d443817055df2ed9124b025b2
Change-Id: I197037b95584e74d443817055df2ed9124b025b2