Pool Package and Class names when writing UsageStats to disk

Write the package and class names at the top of the proto file and
replace each instance of those strings in the rest of the protobuf with
the index of said string.

Some rough number on the impact of this change plus the previous proto
change:
File size on disk reduces to ~13% of XML file size!!!
File read time reduces to ~32-45% of XML read time!
File write time clock at around ~102-107% of XML write time.

Bug: 111422946
Fixes: 111449191
Test: atest UsageStatsDatabaseTest
Change-Id: I6bcce54a2431a964bda2c03bd3be1f3d4b4156e1