Opened some App Links APIs to @SystemApi

This change promotes some of the APIs that Settings uses for the
"Open by default" screen from @hide to @SystemApi.

GTS tests are added in ag/1811536.

This change also changes the protection level for
Manifest.permission.SET_PREFERRED_APPLICATIONS to allow package verifiers
(e.g. the Play Store) to be granted it. This permission is used in the
PM.updateIntentVerificationStatusAsUser() and
PM.setDefaultBrowserPackageNameAsUser() APIs.

Bug:31008483
Test: Patch in ag/1811536 and follow the test instructions there.
Change-Id: I18b069de11eaa8fe97c151fb3cfb63854f1fd056