Add declarations for Android compatibility funcs.

Some parts of Android can't be updated yet so this change adds
declarations (only) for some functions that will be stubbed in
Android-specific code. (That Android-specific code will live in the
Android repo, not the BoringSSL repo.)

Trying to use these functions outside of Android will result in a link
error.

Change-Id: Iaa9b956e6408d21cd8fc34d90d9c15657e429877
Reviewed-on: https://boringssl-review.googlesource.com/2760
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>