WifiManager: startScan throttling

Changes in the CL:
a) Mark startScan() as deprecated so that we can take away this API in
future releases.
b) Modify the documentation regarding scan throttling to indicate that
both foreground & background apps are subject to throttling.

Bug: 68987915
Test: Compiles
Change-Id: I938237b186c39c6b4cfec9dcdbd9b9d196bae0cc