Don't call disconnected AccessibilityServices

An AccessibilityService that doesn't have a connection is
not able to access the resources it needs to respond to
callbacks.

Bug: 65367289
Test: Toggle Select to Speak several times, with and
without TalkBack enabled. No longer crashes. A11y unit
tests and A11yService CTS passes.

Change-Id: I03c647cbaf68d5defcc7fbcec1c746af9968bd35