Adds more test coverage for BrailleDisplay[Controller|Connection].

BrailleDisplayConnection:
- Test when write() is called but the accessibility service is not
  connected.
- Test when write() is called with too large of a buffer.
- Test when write() is called but the FileOutputStream fails to be
  created.
- Test for DefaultNativeScanner#getHidrawNodePaths
- Renamed getDefaultNativeInterface anonymous class to a defined class
  name so that its missing coverage is clearer in dashboards.
  (These instrumentation tests cannot call the native methods.)

BrailleDisplayControllerImpl
- Test connect(UsbDevice) calls the correct system_server method.
- Test when connect is called but the service is not connected.

Bug: 306754563
Test: atest BrailleDisplayConnectionTest
Test: atest BrailleDisplayControllerImplTest
Change-Id: I16fa7eb71dfa2ccf719dbdf7c9a1f167dcd3ae00