adb: split up adb_auth.cpp.

All of the functions in adb_auth.cpp were used in only one of
adb/adbd. Split up them up into adb_auth_host.cpp and adbd_auth.cpp
respectively.

Bug: http://b/29273531
Test: built and flashed bullhead, adb still works
Change-Id: Ib610c5157522634cc273511175152f1306cc52a7
diff --git a/adb/transport.h b/adb/transport.h
index 959681f..aaa8be4 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -108,7 +108,9 @@
         return type == kTransportLocal && local_port_for_emulator_ == -1;
     }
 
+#if ADB_HOST
     std::shared_ptr<RSA> NextKey();
+#endif
 
     unsigned char token[TOKEN_SIZE] = {};
     size_t failed_auth_attempts = 0;
@@ -161,7 +163,9 @@
     // A list of adisconnect callbacks called when the transport is kicked.
     std::list<adisconnect*> disconnects_;
 
+#if ADB_HOST
     std::deque<std::shared_ptr<RSA>> keys_;
+#endif
 
     DISALLOW_COPY_AND_ASSIGN(atransport);
 };