allow CtsPackageWatchdogTestCases to use private api

CtsPackageWatchdogTestCases is statically linked to services.core and
services.net. These two libraries could use the private APIs, so this
test should only be allowed to use private api.

Test: atest CtsPackageWatchdogTestCases:android.packagewatchdog.cts.PackageWatchdogTest#testRegisteringMultipleObservers -- --abi x86_64
Bug: 333776535
Change-Id: I819cb92140c15dd5e0415aaf0c4c3e1d59f6e983