Add conversion constructor for hidl_string.

Test: hidl_test
Change-Id: I61aaba51273ae9c55f77f1a2525df541a95c35fa
diff --git a/HidlSupport.cpp b/HidlSupport.cpp
index 98c235c..ed3e12e 100644
--- a/HidlSupport.cpp
+++ b/HidlSupport.cpp
@@ -37,6 +37,10 @@
     clear();
 }
 
+hidl_string::hidl_string(const char *s) : hidl_string() {
+    *this = s;
+}
+
 hidl_string::hidl_string(const hidl_string &other)
     : mBuffer(const_cast<char *>(kEmptyString)),
       mSize(0),