IMMS: defer showSoftInput until session is established

onStartInput is only dispatched once the session exists. To
avoid still showing the IME on the old window, we need to
defer showing the input until the session has been created
and the input attached.

Fixes: 337766845
Test: atest 'InputMethodStartInputLifecycleTest#testShowingImeDuringSessionChange_doesntShowOnPreviousSession'
Change-Id: I446fa22fbc259a3f3c94be97786a9ef100121ec3