Obfuscate usage stats data stored on disk.

All of the usage stats data stored on disk will now be obfuscated. There
will be a package to tokens mappings file stored on disk which has a
hierarchy of mappings for each string in each package's usage stats
data. A UsageStatsProtoV2 was added to keep the logic clean and separate
from the original usage stats proto parser.

Initial observations show a memory gain of over 60% w.r.t. the usage
stats data size on disk. There is also no performance hit because of this
change - in fact, even with the obfuscation overhead, reads are now over
65% faster and writes are up to 50% faster.

Bug: 135484470
Test: atest UsageStatsTest
Test: atest UsageStatsDatabaseTest
Test: atest UsageStatsDatabasePerfTest
Change-Id: I55ce729033d8b6e4051271802d57c72684053c32