[DataStore] Add UT for Observer

Bug: 325144964
Test: atest SettingsLibDataStoreTest --iterations 1000
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f5eeba5011410f07fe6efe5c2900f16ca13e9e30)
Merged-In: I688d58c27265829b29d65545238cb2d03619acb6
Change-Id: I688d58c27265829b29d65545238cb2d03619acb6
4 files changed