Add explicit cast from size_t to u64.

Fixes compile error on non-64-bit targets.
Fix compile error and warning on Android
Update to latest string-cache.

Includes fix for servo/string-cache#4.
Exclude host libs from Android package.

Fixes startup error caused by packaging the wrong libphf.so.
Merge pull request #2918 from mbrubeck/size_t

Fix build errors for Android (and other 32-bit targets)