Clear liveData when startForRequest

We should not clear the liveData
in onDestory. Because the device list
will be empty when finish the discovery and
the UI will show empty device list.
Instead, we need to clear/reset the livedata
when startForRequest, so we update the devicelist
before the UI pop up.

Bug: 211836059
Test: atest CtsCompanionDeviceManagerCoreTestCases
      atest CtsCompanionDeviceManagerUiAutomationTestCases
      atest CtsOsTestCases:CompanionDeviceManagerTest
Change-Id: I78f3137f5fa1329b52138d221ba7abc38c9f563e