Ensure to create the symlink in the symbols directory

Previous CL dc70fea0 didn't work well
because the rule for creating the symlink is tied to the phony target
com.android.runtime whose output file (com.android.runtime-timestamp) is
never installed unless `m com.android.runtime` is executed.

This change fixes the problem by binding the symlink creation rule to
the apex_manifest.json file that is guaranteed to be installed whenever
the runtime APEX is installed (regardless of the flatteness).

Bug: 120846816
Test: m dist with marlin (flattened) and blueline (non-flattened)
examine symbols.zip file and check that unstripped shared libraries are
found under /apex/com.android.runtime directory
Change-Id: I88b501fac36c11797cffbd1da8687c5f32bdee54