ScanRequestProxy: Add scan throttling

This is a port of the existing scan throttling for background apps in
WifiServiceImpl. We're now going to throttle all external scan requests,
except for requests coming in from apps holding NETWORK_SETTINGS
permission.

TODO: WifiServiceImpl.startScan() should return the status from
ScanRequestProxy.startScan().

Bug: 68987915
Test: Unit tests
Test: Scans from settings UI still works.
Change-Id: Ib388752df554de85ca5c29e424b344ef07a7d5b2
Merged-In: I78e4113317b9835dec6e816f2c5a0ba309d73061
5 files changed