Don't block the boot on hashing sepolicy

onBootPhase seems to run sequentially. Moving the work to a background
thread should fix the boot time regression around 15ms.

Bug: 308471499
Test: enable the flag; atest BinaryTransparencyServiceTest
Test: boot and saw sepolicy hash in the log
Change-Id: I0f3ea9f852eb6a12554e995615bdb9b05478f387