Extract PairingThread out of AdbDebuggingManager

AdbDebuggingManager is a huge, 2k+ lines which is difficult to
understand. By making PairingThread a regular class, we gain in clarity.

Test: NA
Bug: NA
Flag: EXEMPT(refactor)
Change-Id: Ie8873e605d1ccc6250cc612a059ee9cb3c64cff1