Merge IMMS#mMethod{List,Map} into IMMS#mSettings

With this CL the following fields

  InputMethodManagerService#mMethodList
  InputMethodManagerService#mMethodMap

will be effectively merged into

  InputMethodManagerService#mSettings

as immutable fields so that relevant data objects are always
consistent with each other.

There must be no observable behavior change.

Bug: 309837937
Fix: 309870347
Test: atest CtsInputMethodTestCases
Test: atest FrameworksServicesTests:InputMethodSettingsTest
Test: atest FrameworksServicesTests:InputMethodManagerServiceTests
Test: atest FrameworksInputMethodSystemServerTests
Change-Id: I512519ed83b28adb2878945bc3e6eaff9ba29f5e