Fix NPE in TaskPositionerTest

Previously TaskPositionerTest uses the test version of startDrag method,
which does not set mTask member variable.

The CL starts using the real version of startDrag by passing fake task
and IActivityManager.

Bug: 70127718
Test: atest TaskPositionerTest
Change-Id: I62688119eafd8bbc4760236c0f4b453dca74dc80
2 files changed