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/libutils/tests/TestHelpers.h b/libutils/tests/TestHelpers.h
index d8e985e..6801cd7 100644
--- a/libutils/tests/TestHelpers.h
+++ b/libutils/tests/TestHelpers.h
@@ -60,7 +60,7 @@
     int mDelayMillis;
 
 public:
-    DelayedTask(int delayMillis) : mDelayMillis(delayMillis) { }
+    explicit DelayedTask(int delayMillis) : mDelayMillis(delayMillis) { }
 
 protected:
     virtual ~DelayedTask() { }