Add android::base::expected::ok()

This is an alias for has_value() which is meant to be more concise, yet
more explicit than the conversions to bool.

Leave operator bool() in place for now: due to the large number of
users, removal of operator bool() needs to be broken up into incremental
stages.

Test: cd system/core && atest
Test: m
Change-Id: Ib1eb00f47d3c0f2229bb176b62687463b834f280