Expose Secure.get{Int,String}ForUser as systemApi

Bluetooth is going mainline and can no longer use hidden api. We tried
to use the public alternative as our user is the same returned by
contentResolver.getUserId() but errorprone is not happy with it

Bug: 216769091
Test: Build bluetooth as apex mainline module
Change-Id: I195d545cc359d765e261c169032dfad46c916d75
CTS-Coverage-Bug: 217352944