IBinder: support FLAG_CLEAR_BUF

This is @hide and it isn't expected to be very useful in Java, since,
for instance, we would need a corresponding way to clear the data
associated with a Java String object. However, it's here for symmetry.

Bug: 171501998
Test: aidl_integration_test
Change-Id: I9d81f6138c74561b934630a95e86e7e670da5f06