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/libbacktrace/UnwindMap.h b/libbacktrace/UnwindMap.h
index f85b54a..d5bec06 100644
--- a/libbacktrace/UnwindMap.h
+++ b/libbacktrace/UnwindMap.h
@@ -29,7 +29,7 @@
 
 class UnwindMap : public BacktraceMap {
 public:
-  UnwindMap(pid_t pid);
+  explicit UnwindMap(pid_t pid);
 
   unw_map_cursor_t* GetMapCursor() { return &map_cursor_; }
 
@@ -39,7 +39,7 @@
 
 class UnwindMapRemote : public UnwindMap {
 public:
-  UnwindMapRemote(pid_t pid);
+  explicit UnwindMapRemote(pid_t pid);
   virtual ~UnwindMapRemote();
 
   bool Build() override;