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/libmemunreachable/PtracerThread.h b/libmemunreachable/PtracerThread.h
index 4d6ca9a..f88b599 100644
--- a/libmemunreachable/PtracerThread.h
+++ b/libmemunreachable/PtracerThread.h
@@ -32,7 +32,7 @@
 // the parent.
 class PtracerThread {
  public:
-  PtracerThread(const std::function<int()>& func);
+  explicit PtracerThread(const std::function<int()>& func);
   ~PtracerThread();
   bool Start();
   int Join();