Make display unique id stable across x86 and x86_64 builds

... by using a minimal version of CityHash64 for the hash function.
std::hash differs between x86 and x86_64 and also:

"Hash functions are only required to produce the same result for the
same input within a single execution of a program;"

This is problematic for input device configuration files which
reference the display unique id.

Bug: b/186150820
Test: libsurfaceflinger_unittest
Change-Id: If15b66775fa48f14cc56bbd23536b61844b7ae37
Merged-In: If15b66775fa48f14cc56bbd23536b61844b7ae37