Allow a11y services to enter text via the path IMEs use

InputMethodService is the primary and a11y is the secondary.
InputMethodService is not
affected by a11y status. When the session from input method is
established, app can start input (pass input context to input method).
When an a11y session comes back, it will be passed to the app. When
InputMethodManagerService binds to/start input with
InputMethodService, it does the same to a11y services which
requested IME functionalities.

It is possible that input method can edit text before ally sessios are
established. So the EditorInfo passed to a11y could be stale.
So when an a11y session is passed to client, client will send
a notification (input method doesn't have this extra notification) for
the current selection. I think since the time for a11y session establish
shouldn't be long, and we get the current state later, it should be fine
for a11y services.

When input method is disconnected from app (client) (even for input
method switching), we cleared a11y and sessions too. When
input method request sessions, we must rerequest sessions for a11y. This
is mainly because when we unbindCurrentClientLocked(SWITCH_IME), we set
active to false for the current client. Suppose we don't want to
change the current structure of input method, an inactive client
probably should clear accessibility sessions too.

When we switch to a client which already has a session with input
method, there might be some a11y sessions with this client, and
some a11y services might be disabled or enabled while the client was
switched out. We pass unchanged a11y
sessions to client, and request sessions for newly enabled a11y services.
When an a11y service is disabled, it removes its session
from all clients in InputMethodManagerService.

Test: type word through modified "switchToInputMethod". Tested session
notification through logs. Tested client switching, input method
switching, a11y service enabled/disable, multiple a11y services,
a11y service enabled before device reboot.
Also tested work profile.

Bug: 187453053

Change-Id: Ia651a811093a939d00c081be1961e24ed3ad0356