Use system user when installing APKs.

For Mainline module updates, this will effectively be the same operation
as installing the packages as a secondary user, since the packages are
preinstalled and preinstalled packages are visible to users. Also,
Mainline modules cannot be disabled per-user.

Since staged install are only issued by privileged apps having
INSTALL_PACKAGES permission, we believe this change is safe for the sake
of mainline.

Fix: 129397974
Bug: 129744602
Test: repro b/129397974 and verify install succeeds.
Change-Id: I5b388707848b6cdbf20511eea02a0c2abd768b86