Better dumpsys results of activity tasks

Currently only the root task and the leaf task information is
dumped. The intermediate tasks (if any) are not revealed.

Dump all Tasks or TaskFragments in the hierarchy with
proper indent.

Bug: 189384393
Test: dumpsys activity a
Test: dumpsys activity -p $package a
Change-Id: I9a7fde37bae68074237ce3e95bad306323774dfb