Synchronize MediaPreparer host download, attempt 2

Errors extracting the downloaded zip are likely caused by
multiple threads writing/deleting the zip in the same location.
Here we move all write operations into a single static synchronized
class to more cleanly handle these race conditions.

bug: 74124491
Test: run unit tests
Change-Id: Ibe5392f0ed4e98c0563320ed487e5f70468fe5a6
merged-in: I1f2a0b94782d7be1b8cc954c088d84242efe4284
1 file changed