Fixed NPE that crashes Settings.

It happens due to a race condition when the AutofillDeveloperSettingsObserver
is triggered by a settings change while the underlying preferences activity is
being finished.

Fixes: 113030661

Test: echo 'could not reproduce this issue'
Test: runtest --path \
      packages/apps/Settings/tests/unit/src/com/android/settings/core/\
      PreferenceControllerContractTest.java
Test: atest AutofillResetOptionsPreferenceControllerTest \
            AutofillLoggingLevelPreferenceControllerTest

Change-Id: I761195df13ac10705b9ce6b0c7546ec66a85d3ef