Handle autofill auth scenarios where the FillContext cannot be retrieved:

Test: manually modified code to force a null FillContext and verified the
      session is properly cleared (even in the client side)
Test: cts-tradefed run commandAndExit cts-dev -m CtsAutoFillServiceTestCases

Fixes: 65374274

Change-Id: Ica482a48b0be34b89e36f9a34078fdcd48134537