Fix AssistDataRequesterTest

The failure is because mock on ActivityTaskManager does not work.
Although we mock ActivityTaskManager in test case, ActivityTaskManager
in AssistDataRequester uses direct call, instead of mock one.
Make ActivityTaskManager be a member of AssistDataRequester and assign
mocked ActivityTaskManager in test case.

Test: atest AssistDataRequesterTest
Fix: 128560700

Change-Id: I30a075b9a1f483f2986ff43b62adc4304690abac