resourceloader: Simplify getHashMtime() to merely a timestamp

resourceloader: Simplify getHashMtime() to merely a timestamp

Similar to what getDefinitionMtime() does already. No need to
repeat the hash. No need for an array that needs serialising and
unserialising internally.

Change the hash key to avoid using old cache values.

Also moved the comment about hashes being included in the key to
this method. getDefinitionMtime() is a later method that performs
the same logic but doesn't need the entire story again.

Follows-up 044713c4, d3bdda32.

Change-Id: Idd83de5ac27138a2dbf2ec49d81ea9188bd6ad57