/vendor/default.prop is removed.

It is merged to /vendor/build.prop. However, for backwards
compatibility, the old file is read on devices running R or earlier.

Bug: 117892318
Test: TH passes
Test: inspect  /vendor/build.prop and check if it has contents from
the old /vendor/default.prop file
Change-Id: I994cc85fc0765bc9233e9799bcb670a23180f068