defconfig: Set CONFIG_BLK_DEV_LOOP_MIN_COUNT to 16.

To have the kernel pre-create 16 loop devices, which will save boot time
as soon as we start having more than 8 APEXes.

Bug: 119022885
Change-Id: I81fcd421fa3ce69c3f1ae150f7aca3cb64320385
Signed-off-by: Martijn Coenen <maco@android.com>