PROTECTION_FLAG_VENDOR_PRIVILEGED as @TestApi

The flag is used in CTS.

Bug: 70332172
Test: cts-tradefed run commandAndExit cts-dev -m CtsPermission2TestCases
-t
android.permission2.cts.PermissionPolicyTest#testPlatformPermissionPolicyUnaltered

Change-Id: Iba4ce1475991249980cc6a403037f4dd7bf722e5