[nearby] Add CredentialElement class

This class should extend DataElement ideally, but we cannot do this
since the DataElement class as System API is final.

Bug: 290294482
Test: unit test
Ignore-AOSP-First: nearby_not_in_aosp_yet
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:0e9e44c4f06c6ea58e991b695cb34dbc0c9d3b59)
Merged-In: I2ec80b5975a8fb27edb5f92f134d6076738eaac8
Change-Id: I2ec80b5975a8fb27edb5f92f134d6076738eaac8
3 files changed