Make Settings app not use cleartext network traffic.

This CL switches the only two places which use cleartext HTTP in this
app to HTTPS. It also declares in the AndroidManifest.xml that this
app does not use cleartext network traffic, thus asking the platform
and tools to block any such traffic from this app on best effort
basis.

NOTE: The only test that uses cleartext HTTP traffic is in VpnTests.
This test makes cleartext HTTP requests to a third-party service which
does not appear to support HTTPS. Thus, this CL temporarily relaxes
the cleartext traffic policy during this test to keep it working. The
correct longer-term fix for this test is to use a service that offers
HTTPS.

Bug: 19215516
Change-Id: Idf1ff8c66d43d77ef2114b2f1b676927844150e5