adb: switch asocket::enqueue to std::string.

Switch asocket over to taking a std::string instead of apacket* for
data. This allows us to remove asocket specific fields from apacket*.

Test: python test_device.py with x86_64 emulator, walleye
Test: adb_test on host
Change-Id: I9d157ff331a75ba49a54fdd4194e3f6cdff722f4
diff --git a/adb/socket_test.cpp b/adb/socket_test.cpp
index f7c66db..04ad6f3 100644
--- a/adb/socket_test.cpp
+++ b/adb/socket_test.cpp
@@ -114,10 +114,10 @@
     ASSERT_TRUE(s != nullptr);
     arg->bytes_written = 0;
     while (true) {
-        apacket* p = get_apacket();
-        p->len = sizeof(p->data);
-        arg->bytes_written += p->len;
-        int ret = s->enqueue(s, p);
+        std::string data;
+        data.resize(MAX_PAYLOAD);
+        arg->bytes_written += data.size();
+        int ret = s->enqueue(s, std::move(data));
         if (ret == 1) {
             // The writer has one packet waiting to send.
             break;