Increase the default alloc tracker maximum.

Also add a new system property "dalvik.vm.allocTrackerMax" that allows
developers to choose an arbitrary limit.

Change-Id: I246a31d0ee46557544b1babae412246914495f80
Remove an assertion that JDWP packets are small.

Now we support longer allocation tracker runs, it's possible
to collect more than 1MiB of data.

Bug: 9871848
Change-Id: Ifb575538b97563ce909f7e0aa6840ccbd6e93569