Compute SHA256 digest of binaries asynchronously post boot.

Instead of waiting until first invocation of commandline, we will
execute the computation of SHA256 digest of APEXs and Modules in
a separate thread post boot (to not hold up boot time).

This allows the logging of APEX information to statsd to happen
once every reboot. Otherwise, logging may never occur if the users
never make use of the commandline.

Bug: 217469982
Test: Manual. `adb shell cmd transparency get apex_info` no longer
has latency upon first invocation.

Change-Id: I1f09316f5b7cde1795dc49456bae97362e30d5c6