Add external/libconstrainedcrypto to the manifest.

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

default.xml

@@ -181,6 +181,7 @@
   <project path="external/libcap" name="platform/external/libcap" />
   <project path="external/libcap-ng" name="platform/external/libcap-ng" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/libchrome" name="platform/external/libchrome" />
+  <project path="external/libconstrainedcrypto" name="platform/external/libconstrainedcrypto" groups="pdk" />
   <project path="external/libcxx" name="platform/external/libcxx" groups="pdk" />
   <project path="external/libcxxabi" name="platform/external/libcxxabi" groups="pdk" />
   <project path="external/libdivsufsort" name="platform/external/libdivsufsort" groups="pdk" />