Fix AssistDataRequesterTest Flakiness

There's a race condition between setting `mRequestCompleted = true`
and asserts its value as true in #assertReceivedDataCount.

Fix the flakiness by updating `mRequestCompleted` whenever getting
the callback (without handling it asynchronously).

Bug: 280107567
Bug: 296229082
Test: run the test over 200 times
Change-Id: I04dfa87c762fd9ee7dfcb9d5dd5982780f43356c
Merged-In: I04dfa87c762fd9ee7dfcb9d5dd5982780f43356c
(cherry picked from commit 6c112678)