Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/binder/ProcessState.cpp b/libs/binder/ProcessState.cpp
index 5114ac1..4748979 100644
--- a/libs/binder/ProcessState.cpp
+++ b/libs/binder/ProcessState.cpp
@@ -52,7 +52,7 @@
 class PoolThread : public Thread
 {
 public:
-    PoolThread(bool isMain)
+    explicit PoolThread(bool isMain)
         : mIsMain(isMain)
     {
     }