Enable DeviceConfig tuning of scan throttling.

Internally, AppScanStats supports throttling abusive apps which are
scanning aggressively.  This change preserves the current default
values, but enables updating them via DeviceConfig.

Bug: 183625640
Test: atest BluetoothInstrumentationTests
Change-Id: I142c3cf9393fb9eb9b332c243dd72a10da4842b0