Prefer std::string over String8

String8 is deprecated, so use proper C++ std::string instead.
Change DisplayViewport.uniqueId to std::string.
The current usage of String8 in
DisplayViewport hinders refactoring of the code to use viewport types
inside the viewports themselves.
Most of the dependency on String8 is now removed. Once the xml for
properties patch is added, almost all String8 should be gone from the
input system.

Test: atest libinput_tests inputflinger_tests
Bug: 111108021
Change-Id: I580dc27b0449e664a7c9db2cdec1a0c18bf71a09