Update language to comply with Android's inclusive language guidance

See https://source.android.com/setup/contribute/respectful-code for reference

Bug: http://b/161896447
Test: None
Change-Id: I9e5a37a20012b2f7a8eac55701df03f7b7a9ab6c
diff --git a/libutils/Unicode.cpp b/libutils/Unicode.cpp
index b08e061..b6e457b 100644
--- a/libutils/Unicode.cpp
+++ b/libutils/Unicode.cpp
@@ -162,9 +162,9 @@
     if (index >= src_len) {
         return -1;
     }
-    size_t dummy_index;
+    size_t unused_index;
     if (next_index == nullptr) {
-        next_index = &dummy_index;
+        next_index = &unused_index;
     }
     size_t num_read;
     int32_t ret = utf32_at_internal(src + index, &num_read);