Use API for binding service with custom user and executor.

There is actually already a public API overload for
Context.bindService() which allows binding the service with the user
of Context.getUser() and a custom Executor.

We can construct an Executor from our Handler, but this executor is
required to remain the same instance for different calls to
bindService() for the same ServiceConnection by the API, so we should
keep the Executor instance, as well as the Handler instance, instead
of calling getJobHandler() every time. In fact getJobHandler() is
already required to return the same instance, or bindServiceAsUser()
would have thrown the same exception for different Handlers.

Also removed a check in ContextImpl.bindService() to allow mainline
modules to use the bindService() API with Context multi-user.

Bug: 158736025
Test: presubmit
Change-Id: I95717a4a74267dff4fbe1898a82cdfa7f195cc06