ueventd: make parallel restorecon functionality optional

5aa6197d added the ability to
parallelize restorecon to speed up boot for devices that have not
completely moved to genfscon.  This parallel restorecon happens after
the parallel ueventd handling.

This causes a performance regression for devices that have moved to
genfscon, since previously, the restorecon() was done in the main
ueventd thread in parallel with the uevent handlers.

I also tried to run the fully parallelized restorecon in parallel with
the uevent handlers, but that did not make any change to the cold boot
time, likely due to the additional overhead of parallelizing the work.

Bug: 140458170
Test: blueline coldboot time returns to pre-regression time.
Change-Id: I3cd6a869cc9b62792466813d94ad6c69834e854e