adb: switch apacket over to a std::string payload.

Test: python test_device.py with walleye/x86_64 emulator
Change-Id: I0a18941af1cb2279e5019a24ace25741def1202f
diff --git a/adb/adb.h b/adb/adb.h
index c9c635a..a6d0463 100644
--- a/adb/adb.h
+++ b/adb/adb.h
@@ -74,7 +74,7 @@
 
 struct apacket {
     amessage msg;
-    char data[MAX_PAYLOAD];
+    std::string payload;
 };
 
 uint32_t calculate_apacket_checksum(const apacket* packet);