This is only for Conscrypt which always calls the pair in succession. (Indeed
it wouldn't make any sense to not call it.) Remove those two APIs and replace
with a single merged API. This way incomplete EC_GROUPs never escape outside
our API boundary and EC_GROUPs may *finally* be made immutable.

Also add a test for this to make sure I didn't mess it up.

Add a temporary BORINGSSL_201512 define to ease the transition for Conscrypt.
Conscrypt requires https://android-review.googlesource.com/#/c/187801/ before
picking up this change.

Change-Id: I3706c2ceac31ed2313175ba5ee724bd5c74ef6e1
Reviewed-on: https://boringssl-review.googlesource.com/6550
Reviewed-by: Adam Langley <agl@google.com>