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
diff --git a/init/util_test.cpp b/init/util_test.cpp
index 96a5b55..565e7d4 100644
--- a/init/util_test.cpp
+++ b/init/util_test.cpp
@@ -61,8 +61,8 @@
 
 TEST(util, ReadFileSymbolicLink) {
     errno = 0;
-    // lrw------- 1 root root 23 2008-12-31 19:00 default.prop -> system/etc/prop.default
-    auto file_contents = ReadFile("/default.prop");
+    // lrwxr-xr-x 1 root shell 6 2009-01-01 09:00 /system/bin/ps -> toybox
+    auto file_contents = ReadFile("/system/bin/ps");
     EXPECT_EQ(ELOOP, errno);
     ASSERT_FALSE(file_contents.ok());
     EXPECT_EQ("open() failed: Too many symbolic links encountered",