nfc(app): Synchronize usage of NFC vendor callbacks

Also, use a handler to post the vendor notification to avoid blocking
the nfc service thread for Synchronization (AIDL callbacks are marked
`oneway` so they will not block the thread)

Bug: 325896953
Test: atest NfcAdapterTest#testSendVendorCmd
Change-Id: Ib9c0d0b1998a19f9d825224d1372a5622ccedb4f