Clean up android.webkit.update_service_v2.

This flag shipped in V; clean up the old codepaths and remove references
to the flag, but keep the flag itself as it's exported.

Bug: 356086806
Flag: EXEMPT cleaning up launched flag
Test: atest WebViewUpdateServiceTest; atest CtsWebkitTestCases
Change-Id: I324429bc06863575e41a819bbf9e9ff53869fbd3
14 files changed