Add accessibilitySessions to InputBindResult

This way, we can pass back a11y sessions from direct startInput()
return value. And a11yManagerService will only request sessions
from a11y services which the client doesn't have sessions yet
(will not request existing a11y sessions again).

Bug: 187453053

Test: tested manually with talkback. Tested client switching within
"recent apps".

Change-Id: I8efdc9886ce33185a2195b741668c12e319ea660