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.
Merged-In: Ie34acb274696dea999f6309fb914b39ef00f4d77
(cherry picked from commit 843328ff)

Change-Id: I7be4fff7d3fd603b7ab64397a6a05c28f7a34ace