Fixed requestAutofill().

It should behave like notifyViewEntered() but passing a flag, but the current
implementation always start a new session, which is a problem on partitions.

Fixes: 37709350

Test: existing CtsAutoFillServiceTestCases pass
Test: new PartitionedActivityTest.testAutofill4PartitionsManually pass

Change-Id: Ia625a8fa067670e888c2861609ede039abf0f474