Update default value of probe url to be a constant

Default value for probing url should be a constant in
NetworkStack but not an overlayable config due to some
technical limitations. OEMs can mistakenly override configs
that were not designed to be overridden.

Bug: 152730542
Test: atest NetworkStackTests NetworkStackNextTests
Change-Id: I1846958e7c4e8b64ae287718c63e705bb232632a
5 files changed