Non-pixel systemui should also have a separate shell thread

Shell thread should be stable now and is required for slower
devices where systemui eats-up most of the looper time.

Bug: 253378401
Test: tests using aosp sysui (eg. avd_boot_test)
Change-Id: Ia158dd7e005ec3534925d07db1ebfe1b5b3d0d6b