Import more recent changes from google3.

Changes include:

(1) Special handling for invalid examples with empty selection
(http://cr/150640537).

(2) Updates inference code to support sharing embeddings (http://cr/150651395)

(3) Change registration mechanism for SAFT features, to make it more portable
and linker friendly (http://cr/150679054).  For more info, see new comments in
common/registry.h

Test: built code, size didn't change significantly.

Note: code size will go down significantly once I add the -fvisibility=hidden
flag to our Adroid.mk (in a follow-up CL).

Change-Id: I52c19eed1b5111afbd6f4750789295001f7d4aee