libhwbinder: 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: hidl_test + verify by reading memory bits w/ updated kernel
Change-Id: I7dda8f8d24091f77bdaf99a7de446875356c601c
9 files changed