Add an immutable AdditionalSubtypeMap class

This CL introduces AdditionalSubtypeMap, which is immutable can be
used as a data snapshot once instantiated.

This is a mechanical code change. There must be no user observable
behavior change.

Bug: 309837937
Bug: 322062773
Test: atest CtsInputMethodTestCases:InputMethodSubtypeEndToEndTest
Test: atest FrameworksInputMethodSystemServerTests:AdditionalSubtypeMapTest
Change-Id: I5d74c1d7eca07b648241a358b59d88e83bdc146b