Calls setTheme() in onCreate().

This ensures that the theme is applied when the service
first launches, without waiting for the device config
to change.

Before this change enabling the service while the device
was in dark mode would result in dark shortcut font until
the user toggled dark mode off/on.

Bug: 261252772
Test: Enable AccessibilityMenu in Settings while dark
      mode is on; observe light-colored shortcut labels.

Change-Id: Ibc817d1d1fab57ac19f2c9847b95be08c14b1ddb