Add OS Version to Properties

(This is a merge of http://go/wvgerrit/11613 from the Widevine CDM
repo.)

Adds a property for the OS version, implements it on Android, and
adds it to the license request property bag so that Netflix may
use it to discern the supported capabilities of the CDM.

Bug: 18230738
Change-Id: If5174a108093855314f3e0102b83691e20bb247b

libwvdrmengine/cdm/core/include/properties.h

@@ -48,6 +48,7 @@ class Properties {
   static bool GetDeviceName(std::string* device_name);
   static bool GetProductName(std::string* product_name);
   static bool GetBuildInfo(std::string* build_info);
+  static bool GetOsVersion(std::string* os_version);
   static bool GetDeviceFilesBasePath(CdmSecurityLevel security_level,
                                      std::string* base_path);
   static bool GetFactoryKeyboxPath(std::string* keybox);