Use /proc/pid/smaps_rollup when available

smaps_rollup saves CPU time by having the kernel sum smaps fields
before formatting each VMA's smaps; this way, user-space gets a single
dummy smaps entry with pre-summed fields, making it much less
expensive to measure PSS of large processes.

This change makes the Android framework attempt to use smaps_rollup
when available; we fall back to regular smaps when it isn't.

Test: pssbench; see log message about using smaps_rollup
Bug: 63439864
Change-Id: I66e207b02626e3287791e1348f35f58a8d94efe9