Initialize dex cache while holding dex_lock

Fixes multiple threads calling RegisterDexFile occasionally getting
DCHECK failures due to the arrays not being null since the BSS ones
is per dex file.

Bug: 31369621

Test: test-art-host, no DCHECK failure during debug booting

Change-Id: I7b6e4cd03460dd1213eb4e044bdcf5f6103fd5f9