Add -nosystem flag to adb backup

This makes it easy to back up everything that belongs to 3rd party apps, but
nothing that comes with the system per se.  If any system packages are
explicitly named on the command line they will be included in the backup
even if -nosystem was passed.  So, for example, this will back up all 3rd
party apps as well as system settings, but nothing else belonging to
system-deployed apps:

   adb backup -all -nosystem com.android.provider.settings

Bug 5361503

Change-Id: Iebe04b7d7027ca58b9f55e8eb7f219d6cca69269