Install staged APKs before system is ready.

This CL makes the installation happen before
PackageManager.systemReady() completes. This would avoid cases in which
other components started up by SystemServer would query the package
status.

Fix: 123690624
Test: tried the following scenarios:
 - atest apex_e2e_tests
 - adb install --staged file.apk
 - adb install-multi-package --staged file.apk file.apk
 - adb install-multi-package --staged file.apk file2.apk
   file.apex file2.apex

Change-Id: I0d3d5c0695bed8084c1bddcc4fd99575dab00c6f