reupload^2: libsnapshot: set thread priority

It looks like the previous error was due to a missing include that
somehow my LSP didn't catch. The build failure isn't reproducible on
aosp-main but is on git_main. Confirmed this new patch with the #include
builds correctly on git_main.

Read merge thread + worker thread priority from build configurations. In
the case of low memory devices, a lower priority will reduce CPU
utilization post OTA reboot.

Test: th
Change-Id: I7fd398c4688cf78abe738e7320b9f27f5f7d1e13