LOS ANGELES – The King of Pop seemed driven and upbeat in the weeks, even hours, before his death as he rehearsed rigorously for a series of 50 concerts in London that were to begin a late-career comeback. Friends and colleagues said Friday that Jackson appeared in recent months to be rejuvenated by the prospect of performing again. After years of seclusion following a child sex scandal, the pop icon was heavily involved in all aspects of the concert rehearsals. He had hired a personal trainer a