Update tzdatacheck docs, reflect new files location

The tz_version tzdatacheck will now be the one from
the runtime APEX, not /system. The one from /system
will eventually go away.

Test: CTS: run cts -m CtsHostTzDataTests
Test: PTS (cp'd to internal master): run pts -m PtsTimeZoneTestCases
Bug: 116191025
Bug: 119293618
Bug: 113373927
Change-Id: I91e85c25adcdf24e6988bd2ebf7b889f0c254f07