Use IMM#invalidateInput() in EditText#setText() (2nd try)

With this CL, EditText#setText() actually starts using

  InputMethodManger#invalidateInput(),

which does not block the UI thread.

See the previous CL [1] about how that API actually works.

There should be no observable changes from the viewpoint of IMEs.

Note that this CL was once reverted due to Bug 208941904, which was
caused by a misbehaving InputConnection#endBatchEdit() implementation
as a combination of the following bugs:

 * Chromium (crbug.com/1277732)
 * EditableInputConnection (Bug 209958658)

Now those two bugs were addressed.  We have also implemented a
safeguard [2] against the same type of app issues so that the system
can gracefully fall back to the previous behavior as needed.

 [1]: I3161755779080f98bcef0e47dd0c5247d8a3a256
      daa6695c
 [2]: I109e0c26d8249fc2e01323e3e1cb36395fa7cc97
      60c7c55c

Bug: 203086369
Fix: 209008342
Test: atest CtsInputMethodTestCases
Change-Id: I2ce4be729e23ef686e128f832f8cf7debdcd551e