Show the "platform tools" version in adb/fastboot --version.

Annoyingly folks parse the "adb --version" output so we have the
less-interesting protocol version first. But at least now we'll
have the "real" version somewhere...

Bug: N/A
Test: "adb --version"/"fastboot --version"
Change-Id: Ia85b561bd8d84c6fd6995923730d36f53b2f800b