Adds multi_install_skip_symbol_files field (default false) to apex.

Multi-installed APEXes share the same apex_name and are installed
at the same time.

Should be set to true for all multi-installed APEXes except the singular
default version within the multi-installed group.
Only the default version can install symbol files in $(PRODUCT_OUT}/apex,
or else conflicting build rules may be created.

Bug: 199290365
Test: see https://r.android.com/1872018
Change-Id: I0cd9d748d7be3a2de40ed14f13bad71912457ddc