Revert "Rely on IMM#focusOut() to close input session."

This reverts commit 41bb4953.

Ib1b037594ebbb4ad4cf2d59e21c7a8ca9d8dc930 caused Bug 21508503.
Since it looks like a timing issue, many other applications
could be affected potentially.

Bug 21508503 implies that re-enabling IMM#focusOut() globally
requires much more work than we thought.  Probably the memory
leak found in Bug 20820914 should be addressed in a more
conservative way at the moment, e.g., by plumbing events from
ViewGroup to IMMS.

Bug: 21508503
Bug: 20820914
Change-Id: I2228ae0c48ad3d9e0b55875f0dcb5ef8c55b0c5f