handle dynamically set AccessibilityServiceInfo.FLAG_INPUT_METHOD_EDITOR

When the flag is set dynmatically, we need to create the InputMethod()
instance and request to bind ime. If the flag is unset, we clear the
InputMethod() instance and unbind ime.

Fix: 218193835
Test: Tested with talkback. When the flag is not set, we can't use
ime. When the flag is set, we can use ime again (dynamically set
flags).
Change-Id: Ied48d452108a593e8e4ee5c67c3208acbc87335e

Change-Id: I2a85c64b089983807cb7627de9b8512e8f716426
(cherry picked from commit 14e78228)