Add major version code to platform.

It turns the version code into almost a 64-bit integer, with the
new major part being the upper 32 bits.

The only tricky part about this is the backup manager, since it
stored 32-bit version codes in its backup data sets.  This is dealt
with by, when the major version code is not 0, writing MIN_INT as
the version code and following that by the full long version code,
which we can detect when reading.  Note that this makes backup sets
containing apps with major version codes incompatible with older
versions of the platform.

Bug: 64459786
Test: Added in Change-Id: Iab8a682b62103babd6c16a56b8dc1e97d7078658
Change-Id: Ibfffe235bbfcf358b3741abd3f7197fdb063d3f3