Multi-package support for APEX.

This CL introduces two changes to adb install-multi-package:
- If there is at least one apex package in the list of packages, use the
  --staged parameter for both the parent and the child sessions
- When the package being sent is an apex, use the --apex parameter

Bug: 118865310
Test: Printed out the resulting commands and verified that both
non-staged and staged workflow are accepted by PackageManager. Tried
scheduling install sessions for a mix of APK/APEX, only APKs, only APEX.
Change-Id: I8d1a6a7c5408fb95c10d79e38ddaf115a46f5d8b