libconstrainedcrypto, formerly know as libmincrypt, used to be part of
system/core. Moving the code to external/ has a couple benefits:
* The external/ location more accurately reflects the Google-internal
heritage of the code.
* We're planning for removing dependencies on this code for most
consumers in favor of switching to BoringSSL's libcrypto, so it
doesn't make sense to keep the code in system/core.
* Some rare uses of this code will probably remain (e.g. constrained
environments such as bootloaders, TEE, etc.), having the code in a
separate repo makes it simpler to track for projects outside the
main Android tree.
BUG: 27293141
Change-Id: Iedcf78d83390536ebb9b945eeebaf1715f2d4a8e