Add readStrongBinder that takes an interface

We use a template function to let us pass arbitrary interfaces, all of which
can be cast to IBinder.

Change-Id: Iadf21c495cde43e8a5adb85a49e6592196f401ff
Test: unit tests
Bug: 23600713
Signed-off-by: Casey Dahlin <sadmac@google.com>
diff --git a/libs/binder/Parcel.cpp b/libs/binder/Parcel.cpp
index 48bf799..ab2cdab 100644
--- a/libs/binder/Parcel.cpp
+++ b/libs/binder/Parcel.cpp
@@ -1781,10 +1781,15 @@
     return NULL;
 }
 
+status_t Parcel::readStrongBinder(sp<IBinder>* val) const
+{
+    return unflatten_binder(ProcessState::self(), *this, val);
+}
+
 sp<IBinder> Parcel::readStrongBinder() const
 {
     sp<IBinder> val;
-    unflatten_binder(ProcessState::self(), *this, &val);
+    readStrongBinder(&val);
     return val;
 }