Generalize Join to work for any container/element.

This is more scalable than explicitly instantiating templates for the
cross product of containers and element types.

Specifically I'm adding this so I can join an unordered_set in adb.

Change-Id: I0055f3390a0ff26a886a0d41bbf0d4fe3d210f9c
diff --git a/strings_test.cpp b/strings_test.cpp
index 46a1ab5..5f67575 100644
--- a/strings_test.cpp
+++ b/strings_test.cpp
@@ -20,6 +20,8 @@
 
 #include <string>
 #include <vector>
+#include <set>
+#include <unordered_set>
 
 TEST(strings, split_empty) {
   std::vector<std::string> parts = android::base::Split("", ",");
@@ -121,6 +123,17 @@
   ASSERT_EQ(",,,", android::base::Join(list, ','));
 }
 
+TEST(strings, join_simple_ints) {
+  std::set<int> list = {1, 2, 3};
+  ASSERT_EQ("1,2,3", android::base::Join(list, ','));
+}
+
+TEST(strings, join_unordered_set) {
+  std::unordered_set<int> list = {1, 2};
+  ASSERT_TRUE("1,2" == android::base::Join(list, ',') ||
+              "2,1" == android::base::Join(list, ','));
+}
+
 TEST(strings, startswith_empty) {
   ASSERT_FALSE(android::base::StartsWith("", "foo"));
   ASSERT_TRUE(android::base::StartsWith("", ""));