Build: More tombstones in userdebug

For devices using core_minimal, set the number of retained
tombstones in userdebug and eng builds to 50. This will help
with dogfooding.

Bug: 63131375
Test: lunch bullhead_userdebug && m && fastboot flashall && adb getprop tombstoned.max_tombstone_count
Change-Id: I163d40f7a8f73df1af1228309d7a7a95511bdef9