Fix format string warnings

MockCasPlugin.cpp: These are printing out `size_t`s, so we should be
using %zu.

ClearKeyFetcher.cpp: asset_.id() is an `unsigned long long`, so we
can't use PRIu64 for it.

Bug: None
Test: mma. Warnings are gone.
Change-Id: Ie34acb274696dea999f6309fb914b39ef00f4d77