Make IAccessibilityInputMethodSession async even in in-proc cases

This CL mirrors what I've done for IInputMethodSession [1] for A11y
IME, which relies on a corresponding Binder interface
IAccessibilityInputMethodSession.

When the A11y IME and IME client are running in different processes,
this CL literally changes nothing.

 [1]: I56f3198e54f689adc9541f2c2644ee19bcc6deed
      e345e90a

Fix: 236311060
Test: atest CtsAccessibilityServiceTestCases:AccessibilityImeTest#testOnUpdateSelectionForInProcessA11yIme
Change-Id: Ic84fb070e0ff21dffb7e6c9bbdc6320eb5946fd9