wifi: Add support for removing iface

This is needed to support dynamically switching between ifaces
within the same mode.

While there, correct couple of nits:
1. Pass std::string to the |get*IfaceInternal| methods. This makes the
name comparison easier.
2. Update the docstring of |get*Iface| in IWifiChip.hal to indicate that
we'll return ERROR_INVALID_ARGS if the iface name is wrong.

Bug: 33040049
Test: Compiles
Change-Id: I53584013283a10bf109f1c2ffb6136c44f11ed29