Add uprobestats to base_system.mk.

Ignore-AOSP-First: uprobestats currently only exists on the internal branch
Test: m && vendor/google/tools/flashall &&
  adb shell ls /system/bin/uprobestats
Bug: 296108553
Change-Id: I7c801e841e467a7653a076413e9d58b9bc16b1e4