Add InputMethodSubtype#getCanonicalizedLanguageTag()

This is a preparation for implementing Subtype-keyed physical keyboard
layout switching.

This CL introduces an @hide method

   InputMethodSubtype#getCanonicalizedLanguageTag()

so that later CLs can be written by obtaining canonicalized BCP 47
Language Tags from InputMethodSubtype.

See test cases to see how it works.

Bug: 252816846
Test: atest FrameworksCoreTests:InputMethodSubtypeTest
Change-Id: I5276642ba83fb708e95591da9bc7fefe2810b170