Allow CE CDM integrators to specify their own logging

This patch adds a new interface that partners must provide to
Cdm::initialize(), ILogger. ILogger replaces stderr as the sink to which
logging messages are sent. For partners that still want to log to
stderr, a reference implementation that logs to stderr is provided.

As a side-effect of this, many test-related source files had to be
updated to thread the new parameter to Cdm::initialize() through them.
This also necessitated adding a new variant of FormatString() that can
be called with a va_list directly so it can be called from other
functions that take varargs.

Bug: 201446862
Merged from https://widevine-internal-review.googlesource.com/177270

Change-Id: Ie31a10162773883b337f3a6144cf180a2b100139

libwvdrmengine/cdm/util/include/string_format.h

@@ -13,6 +13,11 @@ namespace wvutil {
 #endif
 bool FormatString(std::string* out, const char* fmt, ...);
 
+#ifdef __GNUC__
+[[gnu::format(printf, 2, 0)]]
+#endif
+bool VFormatString(std::string* out, const char* fmt, va_list vlist);
+
 }  // namespace wvutil
 
 #endif  // WVCDM_UTIL_STRING_FORMAT_H_