Installd: new interface to enable apk verity

Note that file-based verity can be enabled only with CAP_SYS_ADMIN.

The implementation is dummy at the moment and waiting for kernel
implementation to be ready.  Having the interface ready first allows our
iteration on the caller side in Package Manager.

Test: Saw logs on install if ro.apk_verity.mode is 1
Bug: 30972906
Change-Id: I5fb03d745111028bafbff1029f480b0386906e8a