Support prefix in SubtypeSwitchingController#dump()

Before making InputMethodSubtypeSwitchingController multi-user aware,
this CL enables prefix support into

  InputMethodSubtypeSwitchingController#dump()

in case we need to adjust the indentation level when each user starts
having its own InputMethodSubtypeSwitchingController instance.

For now there must be no observable behavior change.

Bug: 305849394
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I11b34a99d18a1f4ba65498b2fb1d6acce51ee369