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/liblog/tests/benchmark.h b/liblog/tests/benchmark.h
index 57b3748..e9280f6 100644
--- a/liblog/tests/benchmark.h
+++ b/liblog/tests/benchmark.h
@@ -38,7 +38,7 @@
   Benchmark(const char* name, void (*fn)(int)) : name_(strdup(name)), fn_(fn) {
     BenchmarkRegister(this);
   }
-  Benchmark(const char* name) : name_(strdup(name)), fn_(NULL) {}
+  explicit Benchmark(const char* name) : name_(strdup(name)), fn_(NULL) {}
 
   virtual ~Benchmark() {
     free(name_);