APEX list can be retrieved via the shell command.

When 'list package' command is invoked with '--apex-only' flag, APEXes that
are activated are listead instead of the ordinary APK packages.

Bug: 117589375
Test: `cmd package list packages --apex-only` and
`pm list packages --apex-only` shows apexes
package:com.android.resolv
package:com.android.runtime
package:com.android.tzdata

Change-Id: Ie0f7cc37db0a30e55e2b717be75f5028fbd06ede