Ensure that an explicit GC request initiates a GC.

Previously, if a concurrent GC was in-flight when an explicit GC was
requested a warning would be logged about a recursive GC attempt.  Now,
the requesting thread will block until the GC is inactive and afterward
initiate its foreground collection.

Change-Id: Ibc4cef83315ffe7ccf822a32beb4cce5b1c816bd