[ 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
e51f869190