[memprof] Simplify initialized flags.

As discussed in D145428, the memprof_init_is_running check can be moved
to the end of the initialization routine to avoid intercepting
allocations during initialization. Also, the memprof_init_done flag can
be removed and replaced with memprof_inited. Finally, memprof_inited can
also be moved to the end of the method.

Tested on the existing check-memprof tests; memprof profile collection
succeeded on a large internal workload.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D145528