Fix google-explicit-constructor warnings in system/bt.

* Declare conversion constructors as 'explicit'.
* Add NOLINT to implicit converters.

Bug: 28341362
Change-Id: I81fecf19c0ee2e64dd6778b3dea78e04f111bfb4
Test: build with clang-tidy
diff --git a/service/gatt_server.h b/service/gatt_server.h
index b3d0bbb..f00d294 100644
--- a/service/gatt_server.h
+++ b/service/gatt_server.h
@@ -235,7 +235,7 @@
 
   // Used to keep track of a pending Handle-Value indication.
   struct PendingIndication {
-    PendingIndication(const GattCallback& callback)
+    explicit PendingIndication(const GattCallback& callback)
         : has_success(false), callback(callback) {}
 
     bool has_success;