Automatically unbind for AbstractRemoteService

No need for services to think about unbinding anymore. Now the
AbstractRemoteService counts how many requests are not yet finished and
once the number of unfinished requests drops to zero, schedules an
unbind.

Bug: 117779333
Test: - Started settings which makes many calls to
        PermissionControllerManager. Saw the automatic unbind to eventually
        happen.
      - atest CtsContentCaptureServiceTestCases (with content capture
        enabled and with content capture disabled)
      - atest CtsAutoFillServiceTestCases (4 tests out of WebViewActivityTest fail without and with this change)
Change-Id: Ief2f3512df5a1c55694c0c6b449079a49089bcde