wifi: Move legacy hal types to a new namespace (1/2)

To prevent typename conflicts between the types in
"hardware_legacy/wifi_hal.h" and the HIDL interface,
Move all the legacy HAL types under a separate namespace (legacy_hal).

This is especially needed for Nan data types. Some of the typenames are
exactly the same in the legacy HAL header file and the HIDL interface.

Related changes,
Remove the use of |HalTool| class. This causes compilation failures
because it tries to include wifi_hal.h as well (but, that is now wrapped
inside a namespace).

Bug: 32242225
Test: Compiles
Change-Id: I937877798b81aea280d1797833745ae97fb02dc9