Support for multi-package APK+APEX sessions.

Adding support for single-package staged session containing one APK
only, or multiple-package staged sessions containing an arbitrary number
of APK or APEX packages.

Bug: 118865310
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: I0c018f89705128ecc7e3939cff18eafa19e47a0e