The method's return value is never used, only its side-effects are
used. We rename it to reflect its actual usage, and remove the unused
return value to avoid confusion.
NOTE(mdbooth): The use of global state is a large part of what makes
ImageCacheManager hard to read. I don't mean to imply with this change
that's what should be done here, only to make it clearer to the reader
that it is what's being done currently.
Change-Id: Ic82f6d4f00fd1226673750538019339937a623bc
cb15d34cc1