Use mock intent receiver to handle intent related events instead
of exposing the private methods even for testing only.
The visibility for onPackageAdded, onPackageRemoved and
onUserStarted were exposed for testing only in former commit.
The test coverage for intent receiver part may be missed.
The other intent related methods will be verified in the
follow up commits. This refactor will also help to not to
expose further private methods.
Bug: 230548427
Test: atest VpnManagerServiceTest
Change-Id: If2aa88d8d5c1643e8bd4937673aa792186047fd8
26de58ea79