Upgrade UsageStatsDatabase from XML to Protobuf

Add the relevant methods to read from ProtoInputStream to
various classes.

Also add some framework to handle version changes in
UsageStatsDatabase. There is some risk of users losing all their current
UsageStats data, if something goes horribly wrong. The debug flag and a
keep backup files flag are temporarily set in UsageStatsDatabase with
this change. They will both be unset in the future before the Q release.

Some rough number on the impact of this change:
Proto file size on disk reduces to ~47% of XML file size :)
Proto file read time reduces to ~55% of XML file read :)
Proto file write time increases ~17% over the XML file write :(

There will be a follow up CL to address the file write time regression

Bug: 111422946
Fixes: 111449927
Test: atest UsageStatsDatabaseTest
Change-Id: I084aea796ed2163c42947d52396a36cc7c5562a2