Move AdbKeyStore to its own file outside of AdbDebuggingManager

The AdbKeyStore is moved as it is except for the following changes:

- mKeyStorePersisAction is added and used to send persist message to
  AdbDebuggingHandler to ensure that disk persistance is done
asynchronously.
- mWifiConnectedKeys is changed to CopyOnWriteArraySet to make it
  thread-safe.
- public methods are now package-private.

Test: AdbDebuggingManagerTest
Bug: 420613813
Flag: EXEMPT refactor
Change-Id: Ie6b92eb3490984b3b51071ca68049ad435fe26c9