Wifi usability: Add device mobility state into Wifi usability stats

Device mobility state is helpful for predicting Wifi usability in a
future time conditioned on current Wifi status.
Extend the Wifi.proto to include mobility state.

Bug: 129409568
Test: frameworks/base/wifi/tests/runtests.sh

Change-Id: I40a46bb797e4185720391ff24a943364e81bb00e
Signed-off-by: Mingguang Xu <mingguangxu@google.com>