CachedBluetoothDevice: Rename mVisible to mJustDiscovered

* CachedBluetoothDevice's member mVisible does not mean whether the
  device is visible. Instead, based on its current usage in the library,
  it indicates whether the device was just discovered by SettingsLib.
* Rename the field to mJustDiscovered and associated setters and
  getters.
* This paves way for future addition of mVisible to indicate whether the
  device should be visible to user in the UI.

Bug: 34685932
Test: build only, no functional changes
Change-Id: I616904e6d5bb27dbae74f94819eb0e8607a16e20