FTL: Add variadic concatenation

For now, the arguments can only be integers, string literals, and
truncated string views, as this will mainly be used for tracing.

Bug: 185536303
Test: ftl_test
Change-Id: I3141992c6c9438710f04b056e67a5e8811d39f78
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index 5a80ad0..bc2eb23 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -16,6 +16,7 @@
     srcs: [
         "Flags_test.cpp",
         "cast_test.cpp",
+        "concat_test.cpp",
         "enum_test.cpp",
         "future_test.cpp",
         "small_map_test.cpp",