Create flattened variants for soong apex module

The apexBundle module is mutated flattened variant and unflattened
variant. So we can install flattened apex and unflattened apex at the
same time with adding {Apex module name}.flattened to PRODUCT_PACKAGES.

Bug: 139716748
Test: m -j && Add com.android.art.debug.flattened to PRODUCT PACAGES and
check system/apex/com.android.art.debug directory

Change-Id: I0d8c9a155e47c5d0ff13e2f55d4080d91ef5093d