Run apex_sepolicy_tests for all installed APEXes

This is to check if unknown file context labels are used in APEX. The
reason why this is done in Makefile is to cover vendor-specific SE
policy and prebuilt APEXes. Note that there's already a validation check
when building APEXes (in Soong), which only checks the generic rules,
not vendor-specific.

Bug: 299391194
Test: atest apex_sepolicy_tests_test
Test: m
Change-Id: I4378b7530e8433d2bf188db95cb5bf791e3bf778