Increase PreRebootDexoptJob update engine timeout to 20s

We got stack traces from the field on update_engine error, where most of
them show the cause being "Timed out while waiting for update_engine".

This CL attempts to solve the issue by increasing the timeout.

Bug: 435143755
Test: Presubmit
Flag: EXEMPT bugfix
Change-Id: Iaeebc546e9e603a112f5c53077d9b650e01d992d
1 file changed