Dumpstate: Add tombstone filtering

Only package the ten latest tombstones. This recovers the old behavior
when tombstones were limited to ten by tombstoned, and ensures that
bugreports stay small in size.

It is future work to optimize this, e.g., by packaging as many as possible.

Bug: 64290162
Test: m
Test: adb root && for ((i=0;i<50;i++)) ; do adb shell touch /data/tombstones/tombstone_$i ; done ; adb bugreport test.zip ; unzip -l test.zip | grep tomb
Change-Id: I4072b5fbcf1e0314aa3eebeefbadc61d5ec10787