Clear task in PermissionUsageFragmentTest

This test can fail if this activity had already been open. By passing
the CLEAR_TASK flag, we force the activity to start from scratch.

To reproduce the issue before this fix, run these commands:

    # Open the screen
    adb shell am start -a android.intent.action.REVIEW_PERMISSION_USAGE
    # Run the test
    atest PermissionUsageFragmentTest

Bug: 296473470
Test: see above
Change-Id: I7d0467c987fe9094d68508e76f6c7999a699ff76