use a single file for storing persistent properties

We have seen that storing persistent properties in separate files
causes increased boot latency compared to if they were stored in a
single contiguous file.

This change creates a simple format for a contiguously stored property
file, and adds the support for arbitrary characters in the names of
persistent properties, which previously had been restricted.  It has a
mechanism for converting older devices to the new format as well.

Bug: 64392887
Test: boot bullhead with new properties
Test: boot bullhead and verify old properties are converted to the new
      property file
Test: corrupt property file and ensure that it gets recovered from memory
Test: new unit tests
Change-Id: I60d8201d655ce5c97b33faae81d5ca8dbbb21a14