FTL: Import std::future utilities from SF

Now including docs and expanded tests.

Bug: 160012986
Test: ftl_test
Change-Id: If9eb25646bb33ebc417ea87e6718b46fe0b87cf3
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index 883d138..5bccaca 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -5,6 +5,7 @@
         address: true,
     },
     srcs: [
+        "future_test.cpp",
         "small_map_test.cpp",
         "small_vector_test.cpp",
         "static_vector_test.cpp",