Base64 encoding for string input.

[ Merge of http://go/wvgerrit/119805 ]

This change adds 3 new functions for encoding binary data from a C++
string to a base64 encoded ASCII string.

The CDM and protobuf generated code use C++ strings to store binary
data.  These binary strings are commonly converted into a base64
encoded ASCII string for logging and for returning to the app.

This change also cleans up some of the internal components of the
string_conversions library to use several standard library C++11
method.

Bug: 181732604
Test: CE CDM unittests
Change-Id: I547568c6402e011344260f2df2a06e972122ab8a

libwvdrmengine/cdm/core/src/cdm_session.cpp

@@ -954,7 +954,7 @@ int64_t CdmSession::GetDurationRemaining() {
 
 CdmSessionId CdmSession::GenerateSessionId() {
   static int session_num = 1;
-  return SESSION_ID_PREFIX + IntToString(++session_num);
+  return SESSION_ID_PREFIX + std::to_string(++session_num);
 }
 
 bool CdmSession::GenerateKeySetId(bool atsc_mode_enabled,