[ota-provider-app] support multiple sequential BDX transfers

[holbrookt]

Problem

• ota-provider-app will report an internal error if the user attempts to do a second download after the first has completed
• this occurs because the BdxOtaSender does not re-initialize the TransferSession object after the first transfer has ended (it only calls Reset())

Proposed Solution

• to be more in line with production apps, ota-provider-app should only initialize the BdxOtaSender after receiving QueryImage and determining that BDX is the correct transport method
  • it should also close/deallocate the BdxOtaSender after the transfer has completed, or at least re-initialize an existing instance on the next transfer
  • this is related to [ota-provider-app] Start BDX server after QueryImage #9528