Remove AutofillManagerClient after AutofillManager is finalized

Symptom:
Global reference table overflow happens on system_server.

Root cause:
If a process repeatedly create a context and get AutofillManager
from the context, each BinderProxy of AutofillManagerClient are
registered to AutofillManagerServiceImpl. They aren't removed until
the process is died. BinderProxy consumes global reference and finally
causes crash on system_server.

Solution:
Remove AutofillManagerClient from AutofillManagerServiceImpl after
AutofillManager is finalized.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAutoFillServiceTestCases

Bug: 68747744
Change-Id: I8b76c5368b54317b8696cac470cc390b36e910e7