Change the default background dexopt concurrency to 4.

This changes gives better utilization to have other dexopt tasks
running when one task is blocked on non-parallelizable operations such
as I/O and runtime initialization.

Bug: 307231036
Change-Id: Icd8aa89af0c11dd5f9cba5f0e0671376611385cc
Test: Presubmit
1 file changed