Wifi usability: Add new entry into wifi.proto for metrics collection

Add predictionHorizonSec into Wifi.proto and it is collected together
with Wifi usability score passed from system app.

Bug: 113262380

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

Change-Id: I250a187c08dd9a32cace4aaf9a718d1b232c79b2
Signed-off-by: Mingguang Xu <mingguangxu@google.com>