Check and `speed` compile System UI on boot.

This behavior is controlled by an experimental flag (namespace:
"runtime", name: "dexopt_system_ui_on_boot"). The default is true.

Bug: 227310505
Test: -
  1. Reboot.
  2. See no dex2oat invocation for system UI in logcat.
  3. Install an APEX.
  4. Reboot.
  5. See dex2oat invocation for system UI in logcat.
Change-Id: I7d339d2f3827f0b292e82086dad99a31c31b985a