wifi: Add link layer stats API wrappers in WifiLegacyHal

Adds wrappers over the legacy HAL API for the following:
1. enable/disable link layer stats collection.
2. fetch link layer stats.

Note: Link layer stats structure is quite ugly. The wrapper stucture
declared here (LinkLayerStats) tries to separate out the pointer
elements and ignore the unnecessary variable size elements from them.

Bug: 31991459
Test: Compiles
Change-Id: I7c4188115786542866c7be56cf9f116b3f78e6a3