Throttle the creation of ScheduledExecutorService in Debouncer.

Looks like ScheduledExecutorService creates a thread during
initialization, rather than when the actual action is run, so the
debouncer ends up creating too many threads.

This CL fixes the problem by caching the executor until the actual task
is run.

Bug: 317234694
Test: atest ArtServiceTests
Change-Id: I4d688da7f466ac98c1d4302a390f7aa5aee58830