Introdude IC#deleteSurroundingTextInCodePoints().

This CL introduces a API variant of IC#deleteSurroundingText(), named
IC#deleteSurroundingTextInCodePoints().  Major differences from
the existing one are:
  - The lengths are supplied in code points rather than code units.
  - This API does nothing if there are one or more invalid surrogate
    pairs in the requested range.  (Failure Atomicity)

Note that due to the asynchronous nature of the input method
architecture in Android, implementing the same logic in the input method
side basically ends up with unreliable and unpredictable results.

Bug: 6526420
Change-Id: I7f6a2c5d3d52079ae71623fd5e40d60c688dd5fb