Improve dump format in InputMethodSubtypeSwitchingController

This CL adjusts several prefix handlings in

  InputMethodSubtypeSwitchingController.ControllerImpl#dump(),

which was updated to support IME subtype switching auto mode [1].

Other than cleaning up the dump format, there must be no observable
behavior change in this CL.

 [1]: I71e3a0f4234f829ca6791cd575fda977bc3df13f
      72fa1823

Bug: 311791923
Flag: android.view.inputmethod.ime_switcher_revamp
Test: manually verified with "adb shell dumpsys input_method"
Change-Id: I25d294136d39a31b11efd9afbd3b841dcf2dc713