codec2_vndk: don't spin during sync variable locking/unlocking

Removed spinning and sched_yield for the user-space locking section
of lock/unlock and added comments to the code.

TODO: remove the spinning code in unlock as it does not seem to be
necessary.

Bug: 287432155
Change-Id: Ie283402b3177013f918e3facb287c23a13df0550