Fix -Landroidbp-impl generates file with wrong name.

Bug: 33356875

Test: visualize the output of
    hidl-gen -Landroidbp-impl android.hardware.tests.foo@1.0  -o ~/tmp \
        -randroid.hardware:hardware/interfaces \
        -randroid.hidl:system/libhidl/transport

Change-Id: I9365eb59b4799426371c418cd9863d69c482a5f1
diff --git a/main.cpp b/main.cpp
index e28d187..b7cf4fd 100644
--- a/main.cpp
+++ b/main.cpp
@@ -795,7 +795,7 @@
         ast->getImportedPackages(&importedPackages);
     }
 
-    std::string path = outputDir + "Android.mk";
+    std::string path = outputDir + "Android.bp";
 
     CHECK(Coordinator::MakeParentHierarchy(path));
     FILE *file = fopen(path.c_str(), "w");