adb: Read secure adb keys on every auth request

The framework can now clear the user key list, so we need to reload the
key list on every auth request instead of loading it once when adbd
starts.
This also fixes issues with encrypted devices, where the user key file
is only readable after the user has unlocked the device.

Change-Id: I350c5aab986f8ca86b95f316398d03012553e581