Refactor Bluetooth scan mode APIs

Removed setScanMode(int, long), setScanMode(int mode) is
now a SystemApi. Made getDiscoverableTimeout a public API.
setDiscoverableTimeout is now SystemApi. Discoverable
timeout is now in seconds everywhere.

Tag: #feature
Bug: 195150096
Test: Manual
Change-Id: Iee0a80b382f601a81f14afd7224637b88fbdf7ce