Fix implicit cast from ssize_t to size_t in KeyedVector.h

Fixes a compiler warning for implicit conversion changes from signed
to unsigned which surfaced when refactoring native input libraries.
Add an explicit cast to avoid adding -Wno-sign-conversion compile
flags.

Test: b libsurfaceflinger_unittest
Change-Id: I8866aef7f09ca5173604abe18c586b68bbf12ed6