Offload boot-complete broadcast by default

Move the boot-completion broadcast to the separate "offload" dispatch
queue by default, so that it no longer blocks other user-interactability
broadcast dependencies such as widget lifecycle traffic.

Bug: 208747905
Test: Boot device, observe `dumpsys activity broadcasts`
Change-Id: I26c0966294f50cd42f2c96eb86b606a8f855d552