[nearby] Add discoveryTimestamp field

Test: -m
Bug: 306043245
Ignore-AOSP-First: nearby_not_in_aosp_yet
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:730894831d541ef67063e1d85e3b770476053fe6)
Merged-In: I2151cdeceec1a3180c79b0ea10adb6e5581f4845
Change-Id: I2151cdeceec1a3180c79b0ea10adb6e5581f4845
3 files changed