Decrease default number of partitions from 64 to 10.

...and also added shell cmds to set it so it can be CTS-tested.

Bug: 37997043

Test: manual verification
Test: PartitionedActivityTest.testNoMorePartitionsAfterLimitReached()
Test: CtsAutoFillServiceTestCases pass

Change-Id: I783bd2d3ed248484adeeebd2df425a6304362ad9