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 and takes
java.time.Duration as parameter.

Tag: #feature
Bug: 195150096
Test: Manual
Change-Id: I824b24464987e1db87efc193d30452a6d9d0411e