Add a test to verify IME visiblity when switching focus in split-tasks.

Also, add a new utility methods:

  TestActivity.startSync(
          fromActivity, activityInitializer,activityClass)

  TestUtils.supportsSplitScreenMultiWindow()

for input method tests to be able to launch an activity adjacent from
the source activity with FLAG_ACTIVITY_LAUNCH_ADJACENT flag to enter
split-screen mode (when it supports).

Fix: 275042106
Bug: 228514609
Test: atest KeyboardVisibilityControlTest#\
       testImeHiddenWhenFocusToAppWithStateHiddenFlagInMultiWindowMode
Change-Id: I5b36753459950a13897ba241e41799550932dec1