Cleanup and documentation of ImeTargetWindowState

Following up from the IME layering/input/control target cleanup, this
covers the IME target window on the IMMS side. The name is left more
broad as it represents the target of the InputMethodManagerService,
which will correspond to the IME input target in DisplayContent.

This also passes around some IntDef annotations, and ensures private
ImeTargetWindowState fields are accessed via getters.

Flag: EXEMPT cleanup
Test: atest FrameworksInputMethodSystemServerTests
Bug: 394328311
Change-Id: I50998e59ba78e8778353729bad4f284d0423e6df