Introduce per IME binding data

This is a placeholder for IMMS fields linked to some specific user on
some given display.

This is just a refactoring and cleanup change, no regression is
expected.

Bug: 314149476
Bug: 305849394
Test: atest CtsInputMethodTestCases FrameworksServicesTests
Change-Id: I29864548ea586bc4fe05aef1c8f44b8f66d6e04f