Use order-only dependencies for symbols files

Use an order-only dependency from $(LOCAL_BUILT_MODULE) to the symbols
file so that wiping the symbols directory during installclean doesn't
force anything to rebuild.

Bug: 186507256
Test: m && rm -rf $OUT/symbols && m
Change-Id: Ic164819c71f9db6126ff91c58752c8727cde0d5e