Add LOCAL_SOONG_SYMBOL_PATH

The soong-only variable is used to specify the location where the symbol
file is located. When unspecified, LOCAL_MODULE_PATH is used instead as
before.

This is needed when the path that a file is installed to is different
from the path that the file is accessed from at runtime. For example, if
TARGET_FLATTEN_APEX=true, a file lib/libX.so in an APEX com.android.foo
is installed to /system/apex/com.android.foo/lib/libX.so. However, it's
runtime path is /apex/com.android.foo/lib/libX.so as
/system/apex/com.android.foo is bind-mounted to /apex/com,android.foo.

Bug: 120846816
Test: m and inspect that symbol files exist under
$(PRODUCT_OUT)/symbols/apex/com.android.runtime/

Change-Id: I1b39f6e0cde115d442f14380c365796feff3437b