Remove /system/etc/prop.default

Contents that were in /system/etc/prop.default are now in
/system/build.prop.

The content is placed at the top of build.prop to keep the existing
order between the two files.

Caveat #1: /prop.default still remains for non-Treble devices.

Caveat #2: ReadFileSymbolicLink test was changed to read /system/bin/ps
because /prop.default is no longer guaranteed to exist.

Bug: 117892318
Test: TH passes
Test: inspect  /system/build.prop and check if it has contents from
the old /system/etc/prop.default file

Change-Id: I0d3f96c1656dfe02bfa0e801680f7fa887afd1d9