Restrict getInputMethodWindowVisibleHeight

Make sure only the app currently interacting with the IME can
query this, and restrict the API to apps targeting SDKs before T

Fixes: 204906124
Test: atest 'InputMethodManagerTest#getInputMethodWindowVisibleHeight_returnsZeroIfNotFocused'
Change-Id: If1da19a3dd8c29542afc970b4b201d87547c27a9
Merged-In: If1da19a3dd8c29542afc970b4b201d87547c27a9