Ensure fitSystemWindows is called on the IME at least once.

InputMethodService will recreate the entire layout on config changes.
Until the system insets change again, the new instance will not
hear about the insets.

This change ensures fitSystemWindows is called at least once for all
input method root views that are added to a window.

Bug:11174545
Change-Id: Id0f02f32c0d6e9c2d6b6aeef74a12a13acfbb9e6