Add HardwareBuffer::as_raw for C FFI boundary

Add method to get raw AHardwareBuffer pointer from Rust HardwareBuffer.
Ownership to the buffer is not transferred to the pointer so the client
can still keep the HardwareBuffer instance.

Bug: 362894809
Test: m libnativewindow_rs
Change-Id: Ic960301a0c2c5b6cf08f55078464da845765d10c