FTL: Support ftl::stable_hash based on CityHash64

Extends stable hash from libui by increasing maximum string length
supported from 16 to 64 bytes (CityHash64 v1.0.1 - please see
http://code.google.com/p/cityhash/).

Bug: 185536303
Bug: 194863377
Test: ftl_test
Test: Compare with 64-bit std::hash<std::string_view>
Change-Id: Ib123d0690a0d2150aa51c91526784dcf77a65d46