Move remaining logic of ImeInsetsSourceConsumer to InsetsSourceConsumer

Also doing some minor adjustments for preparation of the ImeStatsToken
refactoring.

Test: atest CtsInputMethodTestCases
Flag: android.view.inputmethod.refactor_insets_controller
Bug: 298172246
Change-Id: I51fb25ad8f402dd0834778835008df03ae12ab88