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/HeapWalker.h b/libmemunreachable/HeapWalker.h
index 3c1b513..b25696f 100644
--- a/libmemunreachable/HeapWalker.h
+++ b/libmemunreachable/HeapWalker.h
@@ -48,7 +48,7 @@
 
 class HeapWalker {
  public:
-  HeapWalker(Allocator<HeapWalker> allocator) : allocator_(allocator),
+  explicit HeapWalker(Allocator<HeapWalker> allocator) : allocator_(allocator),
     allocations_(allocator), allocation_bytes_(0),
 	roots_(allocator), root_vals_(allocator),
 	segv_handler_(allocator), walking_ptr_(0) {