Handle the race condition when calling uncrypt services.

We call uncrypt services to setup / clear bootloader control block (BCB)
for scheduling tasks under recovery (applying OTAs, performing FDR).
However, we cannot start multiple requests simultaneously. Because they
all use the same socket (/dev/socket/uncrypt) for communication and init
deletes the socket on service exits.

This CL fixes the issue by a) adding synchronized blocks for the service
requests; b) checking the availability of the socket before initiating a
new one.

Note that adding synchronized blocks to RecoverySystem doesn't help,
because the calls could be made from different processes (e.g. priv-app,
system_server).

Bug: 31526152
Test: FDR works while a priv-app keeps calling clear BCB.

Change-Id: I95308989e849a9c98a9503ac509f2bc51ed3de19
(cherry picked from commit 794c8b0b)