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 Merged-In: I1846958e7c4e8b64ae287718c63e705bb232632a
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