Ensure metadata bundles are unparceled before making them available

Bundles can't be read from multiple threads safely. This adds locking
around a read that had been previously overlooked and ensures that
unparcel is called on the metadata before it is even available to
other threads.

bug:17894033
Change-Id: I9a4b86a0d0af05b1dcba28a52df2e7a87c685704