init: do not load persistent properties from temporary /data

With full disk encryption, a temporary /data partition is mounted to
start a minimum subset of the frameworks.  Later, once /data can be
decrypted it is mounted again.  load_persist_props is called both when
the temporary /data partition is mounted and again after the real
/data is mounted; this is a mistake.

This change checks to see if we're a FDE device and if so, returns the
first time load_persist_props is called.

Test: boot bullhead (FDE) with and without boot pin and check that
      persistent properties are loaded
Test: boot sailfish (FBE) and check that persistent properties are loaded
Change-Id: I6ed725072bdb27d80bfa6575d0a4876b08c6a4bc