[2/3] Install persistence support

Add persist.sh to save and restore data across installs.  Initial data
includes property ro.sf.lcd_density and file etc/hosts.  Behavior is
controlled via configuration file etc/persist.conf.

Change-Id: I946d620e06a8dc63a6abec862a7517f8b458f2d1
apns update

Change-Id: I8affd7798f6564f25c664e2312a401c8ef78d9ff