Fix potential build break

When BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED is enabled, the build system
may attempt to create a symlink in TARGET_ROOT_OUT before TARGET_ROOT_OUT
is created. Also make the symbolic link depend on
INSTALLED_DEFAULT_PROP_TARGET so that the symbolic link is properly created
when only bootimage is made.

Bug: 63645797
Test: on non-A/B device, make installclean && make bootimag and then verify
      that the symbolic link exists at $(TARGET_ROOT_OUT)/default.prop
Change-Id: Ied7b32abd7a5fd5456c4cb5ff68a56debe6254b7
(cherry picked from 58c1b042)