Reland "libutils: Introduce StaticString16""

This time with old branches excluded.

Bug: 138856262
Test: Run unit tests.
Change-Id: Id0bb1d54b71e38244d64f1b684db1fda81de854c
Merged-In: I8da93f2c9b95183e32d4a2ea895f90c449abbe4d
diff --git a/libutils/SharedBuffer.h b/libutils/SharedBuffer.h
index fdf13a9..476c842 100644
--- a/libutils/SharedBuffer.h
+++ b/libutils/SharedBuffer.h
@@ -102,7 +102,12 @@
         // Must be sized to preserve correct alignment.
         mutable std::atomic<int32_t>        mRefs;
                 size_t                      mSize;
-                uint32_t                    mReserved[2];
+                uint32_t                    mReserved;
+public:
+        // mClientMetadata is reserved for client use.  It is initialized to 0
+        // and the clients can do whatever they want with it.  Note that this is
+        // placed last so that it is adjcent to the buffer allocated.
+                uint32_t                    mClientMetadata;
 };
 
 static_assert(sizeof(SharedBuffer) % 8 == 0