libbinder: support TF_CLEAR_BUF

This flag instructs the kernel to clear transactions from send/reply
buffers for certain transactions which may contain sensitive data, as a
security precaution.

Bug: 171501998
Test: binderLibTest (only checks there is no error)
Change-Id: I69d732e5cc2b15472dc52d47c66716bc1b3be6ae
diff --git a/libs/binder/tests/binderLibTest.cpp b/libs/binder/tests/binderLibTest.cpp
index 98f0868..ad4729d 100644
--- a/libs/binder/tests/binderLibTest.cpp
+++ b/libs/binder/tests/binderLibTest.cpp
@@ -402,6 +402,14 @@
     EXPECT_EQ(NO_ERROR, ret);
 }
 
+TEST_F(BinderLibTest, NopTransactionClear) {
+    status_t ret;
+    Parcel data, reply;
+    // make sure it accepts the transaction flag
+    ret = m_server->transact(BINDER_LIB_TEST_NOP_TRANSACTION, data, &reply, TF_CLEAR_BUF);
+    EXPECT_EQ(NO_ERROR, ret);
+}
+
 TEST_F(BinderLibTest, Freeze) {
     status_t ret;
     Parcel data, reply, replypid;