Clarified how to regenerate debug.keystore

Clarified that you must both build and run to regenerate a
missing/expired debug.keystore file (have tested that this fix works
on Linux and Mac), removed information about troubleshooting for
non-Gregorian calendars because the original issue has been marked as
outdated and is no longer a problem.
(https://code.google.com/p/android/issues/detail?id=834)

Bug: 18064192
Change-Id: I1235fa14044e388ccf578beebfc77349eb6205cf