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
Test: Presubmit
(cherry picked from https://android-review.googlesource.com/q/commit:350d7731a8e87bfca87611a7e420ccd6320be4ab)
Merged-In: Icd8aa89af0c11dd5f9cba5f0e0671376611385cc
Change-Id: Icd8aa89af0c11dd5f9cba5f0e0671376611385cc
1 file changed