[Offload] Fix race condition in DiscoveryProviderManager

https://docs.google.com/document/d/1bHvmXFM84JRXH2IIe_ZI3LcIL-muRFgePafkBBSg9sM/edit?resourcekey=0-AHMzhjnO83DKHC74P2JW_A#

Test: Unit test
Ignore-AOSP-First: nearby_not_in_aosp_yet
Fix: 255922206
Change-Id: I9cfcfcf503c4e3d813a4959ca291d29f1290a799
6 files changed