Unlock should always wait for pending PRE_BOOT.

While processing an unlock request, we might go async to handle
long-running operations like dispatching PRE_BOOT_COMPLETED.  This
change ensures that all unlock requests for a particular user wait
in line behind any pending async operations.

Without this CL, any subsequent unlock requests would immediately
return successful, even though PRE_BOOT_COMPLETED events were still
being processed.

Bug: 28240584
Change-Id: I307d6aaebfb8f38028f3666a2e19e4399b7cf3a7