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/Allocator.h b/libmemunreachable/Allocator.h
index a8f579e..5390739 100644
--- a/libmemunreachable/Allocator.h
+++ b/libmemunreachable/Allocator.h
@@ -109,13 +109,13 @@
   }
 
   // Construct an STLAllocator on top of a Heap
-  STLAllocator(const Heap& heap) :
+  STLAllocator(const Heap& heap) :  // NOLINT, implicit
       heap_(heap) {
   }
 
   // Rebind an STLAllocator from an another STLAllocator
   template<typename U>
-  STLAllocator(const STLAllocator<U>& other) :
+  STLAllocator(const STLAllocator<U>& other) :  // NOLINT, implicit
       heap_(other.heap_) {
   }
 
@@ -155,12 +155,12 @@
  public:
   ~Allocator() {}
 
-  Allocator(const Heap& other) :
+  Allocator(const Heap& other) : // NOLINT, implicit
       STLAllocator<T>(other) {
   }
 
   template<typename U>
-  Allocator(const STLAllocator<U>& other) :
+  Allocator(const STLAllocator<U>& other) :  // NOLINT, implicit
       STLAllocator<T>(other) {
   }