Clarify two-phase constructions on two IME-related controllers

This CL aims to clarify that the following controllers have relied on
two-phase construction, and their default states are just empty.

 * InputMethodSubtypeSwitchingController
 * HardwareKeyboardShortcutController

While this CL looks to be a behavior change, in reality these two
controllers have relied on subsequent update() calls to be fully
initialized for each user. Thus there must be no user observable
behavior change.

Bug: 305849394
Bug: 347693610
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Ib3834fb7b9fcd87b5ca8cb4855a0aa7b9a524ecf