Default to all users in pm list packages shell command

Change the default user from USER_SYSTEM to USER_ALL users as it is more
intuitive on multi-user form factors. If a -U option is given then all uids
are listed for USER_ALL. Ex:
    $ adb shell pm list packages -U com.android.systemui
    package:com.android.systemui uid:10145,1110145

Bug: 146343680
Test: existing unit tests still pass
Change-Id: I814c5c327752ae1e14882c284461b2edff710dae