Add a shell command for IMMS to test instant apps

This change adds a shell command to allow InputMethodManagerService
(IMMS) to bind to InputMethodService provided by instant apps, like we
did so for AccessibilityManagerService [1].  Note that binding to an
InputMethodService provided by instant apps is not a supported
scenario.  In theory we can avoid doing this by having a separate APK
for MockIme instead of putting everything into
CtsInputMethodTestCases.apk but that configuration is not yet
supported by our test harness hence it doesn't work right now hence
having this special mode for testing is the only way we have right
now.

 [1]: Ifced735a9a6e495747372dd8b00fdd64933a09c7
      d223db31

Bug: 79484568
Test: Manually verified as follows:
  1. Build a test IME that has android:targetSandboxVersion="2" in its
     AndroidManifest.xml.
  2. Install the test IME APK with 'adb install --instant <APK>'
  3. adb shell ime list -a -s
  4. Make sure that the test IME is not in the list.
  5. adb shell cmd input_method set-bind-instant-service-allowed true
  6. adb shell ime list -a -s
  7. Make sure that the test IME is now in the list.
  8. Select that test IME.
  9. Make sure that the test IME is working.
 10. adb shell cmd input_method set-bind-instant-service-allowed false
 11. Make sure that the test IME is no longer the current IME.
 12. adb shell ime list -a -s
 13. Make sure that the test IME is no longer in the list.
Test: cts-tradefed run cts-instant -m CtsInputMethodTestCases
Change-Id: I4383129fd9e229a849282e874aff5d4eef1f49f8