Determine local mtu for offload LE socket

The local MTU is selected as the minimum of:
  - The socket hal's offload capabilities
  - The application's requested maximum RX packet size

However, the MTU must be at least the minimum required by the L2CAP LE specification.

Bug: 342012881
Bug: 367419086
Test: m -j
Change-Id: Ia6360280b0527cb1218701d99a4ca424e38d83c9