Add /system_ext/apex to dirs for builtin-apexes

/system_ext partition is available for most targets, and GSI is going to
the partition to install extra VNDK apexes.

Bug: 140166582
Test: m && boot && check if apex in system_ext/apex is activated
Change-Id: I8242a821bf57981f6389dd4b996806b9e0c4d511
1 file changed