Clarify on DEFAULT_INPUT_METHOD and InputMethodInfo#getId()

This CL aims to clarify more on what "IME ID" means in Android.

"IME ID" is a string representation of a certain implementation of
InputMethodService, which has always been computed by

  new ComponentName(packageName, serviceName).flattenToShortString(),

then has been exposed to developers through at least the following two
public APIs.

 * Settings.Secure.DEFAULT_INPUT_METHOD
 * InputMethodInfo#getId()

Now that our IME CTS tests are already requiring that "IME ID" equals
to test IMEs' component name indirectly, it would be more beneficial
if we clearly mention that "IME ID" is guaranteed to be constructed by

  ComponentName#flattenToShortString()

hence is also guaranteed to be parsed with

  ComponentName#unflattenFromString().

It is to late to change strong representations that have been
exposed to developers for more than 10 years anyway.

Other than updating public API documents, this CL does nothing.

Fix: 192392438
Test: presubmit
Change-Id: I013cd79b92363fe6e438f445606b5fc53e2660f6