Support ATSC license installation

[ Merge of http://go/wvgerrit/163900/ ]

ATSC licenses can be saved by calling
MediaDrm#setPropertyString("storeAtscLicense",<value>)
where <value> is
"<atsc-key-set-ID>:<license-file-data in Base64 format>"

Before storing an ATSC license a session must be opened and the
ATSC mode must be enabled.

Use MediaDrm#setPropertyString("atscMode","enable");

Bug: 176871821
Test: WV Unit/integration/Luci tests
Test: libwvdrmdrmplugin_hal_test
Test: GtsMediaTestCases
Change-Id: Iec2a8b7f87b1122395d06856202278b92316fdfe

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -235,6 +235,10 @@ class CdmEngine {
   virtual CdmResponseType RemoveOfflineLicense(const std::string& key_set_id,
                                                CdmSecurityLevel security_level);
 
+  virtual CdmResponseType StoreAtscLicense(
+      RequestedSecurityLevel security_level, const CdmKeySetId& key_set_id,
+      const std::string& serialized_license_data);
+
   // Usage related methods for streaming licenses
   // Retrieve a random usage info from the list of all usage infos for this app
   // id. If |error_detail| is not null, an additional error code may be provided