Introduce multi-client IME for special form factors

An advanced multi-display support is requested for certain Android
form-factors so that user(s) can type text on each display at the same
time without losing software keyboard focus in other displays. This is
not possible in existing Android IMEs that are built on top of
InputMethodService class, because the assumption that a single IME
client can be focused at the same time was made before Android IME
APIs were introduced in Android 1.5 and many public APIs in
InputMethodService have already relied heavily on that
assumption. Updating InputMethodService class to support multi-client
scenario is, however, quite challenging because:

 1. doing so would introduce an unacceptable amount of complexity into
    InputMethodService, which is already hard to maintain,

 2. IME developers still need to update their implementation to be
    able to support parallel requests from multiple focused IME
    client, which may require non-trivial redesign in their side
    (e.g. input decoder, typing history database, ...), and

 3. actual use cases for multi IME clients are expected to be evolved
    rapidly hence the new protocol is not yet stable and not yet ready
    to be exposed as public APIs.

This is why a new type of IME needs to be designed and developed
specifically for such special multi-display environments, rather than
reusing existing InputMethodService public class.

Note that there must be no behavior change unless multi-client IME is
explicitly enabled with 'adb shell setprop', which requires root
permission.

See multi-client-ime.md for details.

Fix: 114662040
Test: Manually verified as follows:
  1. make -j MultiClientInputMethod
  2. adb install -r $OUT/system/priv-app/MultiClientInputMethod/MultiClientInputMethod.apk
  3. adb root
  4. adb shell setprop persist.debug.multi_client_ime \
       com.example.android.multiclientinputmethod/.MultiClientInputMethod
  5. adb reboot
  6. Try multiple text input scenario
Change-Id: I41dfe854557b178d8af740bc2869c936fc88608b