Reuse mHandler AdbKeyStore to fetch paired devices

AdbKeyStore is thread-safe, so it's safe to re-use the instance. Reusing
the instace makes sure to get the updated in-memory AdbKeyStore state
without the need to re-read the files which might haven't been persisted
yet.

Test: AdbDebuggingManagerTest
Bug: 420613813
Flag: EXEMPT bugfix
Change-Id: Ia55852559c7d1bc07f9501ba0064f22d846831d9