libchromeos: Unify interface for SecureBlob to match that of Blob

Streamlined the interface of SecureBlob to match that of Blob, which
is just a vector<uint8_t>. I have kept a couple of custom functions
to inter-operate with std::string.

The change greatly simplifies the usage of SecureBlob class as it
does not require as many type casts and other weird things, and in
addition the functions are pretty much those from std::vector, so
very familiar to people

BUG=None
TEST=`FEATURES=test emerge-link libchromeos chaps cryptohome privetd webserver update_engine`

Change-Id: Ifb82a2ba7fe3e8f8bdef4898d929372ee4a75476
Reviewed-on: https://chromium-review.googlesource.com/262479
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Utkarsh Sanghi <usanghi@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
4 files changed