Merge constructor and start in ScanManager

This helps clean up code inside ScanManager. The start function has only
one reference inside GattService.start(), where it is called right after
the constructor. Therefore, merging the start logic into the constructor
seems reasonable.

Bug: 301345556
Test: atest BluetoothInstrumentationTests
Change-Id: I3a97bde6982038189aa2ba78634a7f6fc2929ed9