Autofill: Shell command for saved password count

This is for testing. Will be replaced with a TestAPI in the future.

Bug: 184396750
Test: adb shell cmd autofill get saved-password-count 0
Test: atest CtsAutoFillServiceTestCases:SavedDatasetsInfoTest
Change-Id: I6560b34e085bb36dc21d22479141b1310326c91b