Refactored AbstractRemoteService in 2 classes.

AbstractRemoteService was spun off from RemoteFillService, which only supports
1 pending request while not bound to the service - if a new request comes,
it cancels the previous one.

This behavior is fine for Autofill, but for Content Capture all requests must be
queued. In fact, the upcoming CTS tests for Content Capture were failing because
the 1st PendingOnContentCaptureEventsRequest would cancel the pending
PendingSessionLifecycleRequest.

So, this CL fix this problem by creating 2 subclasses:
 - AbstractSinglePendingRequestRemoteService
 - AbstractMultiplePendingRequestsRemoteService

Test: atest CtsContentCaptureServiceTestCases CtsAutoFillServiceTestCases

Bug: 111276913
Bug: 117779333

Change-Id: I43bb98be16f5def037f85a415e1f77799adf156e