Fix google-explicit-constructor warnings in system/core.

* Declare explicit conversion constructors.
* Add NOLINT for implicit conversion constructors.
* Fix also some misaligned indendations.

Bug: 28341362
Change-Id: Idf911f35923b408d92285cc1a053f382ba08c63e
Test: build with clang-tidy
diff --git a/fastboot/socket.h b/fastboot/socket.h
index de543db..7eaa0ab 100644
--- a/fastboot/socket.h
+++ b/fastboot/socket.h
@@ -104,7 +104,7 @@
 
   protected:
     // Protected constructor to force factory function use.
-    Socket(cutils_socket_t sock);
+    explicit Socket(cutils_socket_t sock);
 
     // Blocks up to |timeout_ms| until a read is possible on |sock_|, and sets |receive_timed_out_|
     // as appropriate to help distinguish between normal timeouts and fatal errors. Returns true if