Merge constructor and start in multiple managers

This helps clean up code inside multiple managers. Their start functions
have only one reference each inside GattService.start(), where they are
called right after the constructors. Therefore, merging the start logic
into the constructor. Also fixing some wrong documentations in
PeriodicScanManager.

Bug: 301530940
Test: atest BluetoothInstrumentationTests
Change-Id: I63d540e2f4afb6a8e5441e4b8e552f5729d7bda5