Fixing A11yServiceInfo.packageNames set to Null not work

When setting new package names from A11y services, these package names
only added into the old ones, but didn't be removed old package names
and cleared if the updated package names is Null. Doing the clear
operation for old package names and adding the new ones if there's
any new package names.

Bug: 158314907
Test: a11y CTS & unit tests
Test: manual testing with reporter's test APK
Change-Id: I753a1a2c8909f767a1f7089e6e98e8ad856c09af