Allow a service to override another.

For instance, on vendor.img:
service foo /vendor/bin/nfc
    ...

And then on odm.img:
service foo /odm/bin/super-nfc
    override

Allows a service on ODM to override a HAL on vendor.

Bug: 69050941
Test: boot, init_tests
Change-Id: I4e908fb66e89fc6e021799fe1fa6603d3072d62a