Configurations/15-android.conf: detect clang by PATH, not by CC.

Since they intend to omit gcc, it's more appropriate to simply detect
if there is NDK's clang on PATH, as opposite to requiring to specify it
with CC=clang (and looking for it on PATH).

Also detect NDK version and default to armv7-a for NDK>16.

Address failure to recognize -D__ADNDROID_API__=N in CPPFLAGS.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5613)