Don't export storeKey(), and update comments

storeKey() is no longer used outside KeyStorage.cpp, so make it a static
function.  Also fix the documentation for storeKey() (e.g. it's no
longer safe to directly move/rename directories created by storeKey() --
one must use RenameKeyDir() instead).

No functional changes.

[ebiggers@ - cleaned up slightly from satyat@'s original change]

Bug: 190398249
Change-Id: I85918359e77bef414dfddfe5ded30fcde6514013
2 files changed