supplicant(interface): Minor fixes in P2P iface

Fixing couple of issues found in the interface during implementation.
1. WpsProvisionMethod::KEYPAD is not used in wpa_supplicant core. It is
only used inside Android framework. So, remove them.
2. Added |setDisallowedFrequencies| method.
3. Changed the pin type to string from vector of bytes because that is
used by Android framework and wpa_supplicant.

Bug: 31497295
Test: Compiles
Change-Id: I69db530e23c2d3af907c3a5c824cf8291cf12c35