Don't overwrite persistent property files

This fixes a bug introduced by I81b6bd984aad8f7ddec93ce74f4543e4f71be508

In the original CL, setting a sysprop `next_boot.<name>` and then
rebooting the device could remove all the persistent properties stored
in /data/property/persistent_properties. It happened because the
function `WritePersistentProperty` is called with a properties set which
is initialized as an empty set and then added with the properties which
had the next_boot prefix.

As a result...

Before the boot:
* persist.a = 1
* next_boot.b = 2
* next_boot.persist.c = 3

After the reboot:
* b = 2
* persist.c = 3

persist.a gets lost.

This change fixes the issue by populating properties set from the memory
and then save it to the file.

Bug: 306062513
Bug: 300111812
Test: do the following.
$ adb root
$ adb shell setprop persist.a 1
$ adb shell setprop next_boot.b 1
$ adb shell setprop next_boot.persist.c 1

$ adb reboot
$ adb shell getprop persist.a
1    // was (none) before this change
$ adb shell getprop b
1
$ adb shell getprop persist.c
1

$ adb reboot
$ adb shell getprop persist.a
1    // was (none) before this change
$ adb shell getprop b
     // (none) because b isn't persisted. WAI.
$ adb shell getprop persist.c
1

Change-Id: I85d3777f9b32523b010e49b8ca53f4319dd2ce05