Update protobuf library from 2.3 to 2.6.

The dependency here can be removed because it it automatically added
when LOCAL_PROTOC_OPTIMIZE_TYPE=nano by
build/core/host_java_library_common.mk.

Change-Id: I949d32fb5126f1c05e2a6ed48f6636a4a9b15a48
Signed-off-by: Chet Kener <Cl3Kener@gmail.com>