Support Data Saver awareness in libcom.android.tethering.dns_helper.so

Make ADnsHelper_isUidNetworkingBlocked() to reference 'metered'
information and Data Saver related BPF maps to make the final decision.

Bug: 288340533
Test: atest dns_helper_unit_test
Change-Id: I51b1dadd56a8d6fda3f8b18d64740e52b76e1bfe