[nearby] Sync discoveryTimestamp field name

Test: -m
Bug: 306043245
Ignore-AOSP-First: nearby_not_in_aosp_yet
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d6acb0655f4fa619bd0ff7a6e7a1a76959db29d9)
Merged-In: I96e03660dbd49f361a436276d62473504f88d657
Change-Id: I96e03660dbd49f361a436276d62473504f88d657
3 files changed