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/init/parser/tokenizer.h b/init/parser/tokenizer.h
index 8312a08..ade8f73 100644
--- a/init/parser/tokenizer.h
+++ b/init/parser/tokenizer.h
@@ -36,7 +36,7 @@
 // a TOK_NEWLINE will not be generated for that line.
 class Tokenizer {
  public:
-  Tokenizer(const std::string& data);
+  explicit Tokenizer(const std::string& data);
   ~Tokenizer();
 
   enum TokenType { TOK_START, TOK_END, TOK_NEWLINE, TOK_TEXT };