Clear the inset of previous IME when necessary.

In the following scenario, the system continues keeping the previous
IME's inset in the display until the new IME shows its window.
1. an IME shows its software keyboard with reporting non-empty inset.
2. the user switches to another IME, which does not show the software
   keyboard when it is selected (e.g., because the physical keyboard
   is connected.)

This CL tries to address above situation by introducing a new state
flag IMMS#mShouldClearInsetOfPreviousIme to clear the previous IME's
inset when necessary.

Bug: 15922840
Change-Id: Ib04967f39b2529251e4835c42e9f99dba2cf43f2