init: Introduce thread pool to Coldboot

This commit introduces thread pool to Coldboot. Current implementation
forks multiple child processes to handle uevents and distributes the
equal numbers of the uevents to each process. As it does not reflect the
actual workload of each uevent handling, it tends to overload one or two
subprocesses, which hurts the overall performance. Introducing
threadpool resolves this issue since the workload is evenly distributed
across threads.

Thread pool based implementation of Coldboot is hidden behind the
feature flag `enable_coldboot_threadpool`.

Flag: com.android.ueventd.flags.enable_coldboot_threadpool
Bug: 421025789
Test: DUT boots
Change-Id: I39c35c0654904f522f126c5aa6581099448f8a94