binary: Allow specifying a path for generated proto headers

protobuf compilation defaults to dropping the generated objects and
headers onto a path derived from the origin source path. This poses
a problem with some of our repositories since they're not placed in
their original paths, and may include hard references to pb headers
assuming the original directory.

Typical use of this will be to place a

LOCAL_PROTOC_OUTPUT := <original repo path>

line in the relevant module block (for example, "hardware/ril" for our
"hardware/ril-caf")

Change-Id: Ib9b0cadd47e29a0d2e693b331cfc18e71b048ee5