Add VintfObject::getKernelLevel.

Remove the unused API VintfObject::getCompatibleKernelRequirements and
add a more useful VintfObject::getKernelLevel. The new API
returns the kernel FCM version specified in the device manifest.

For example, if a device runs kernel from 4.19-r branch
(with kernel version 4.19.y), then this API should return
Level::R.

Old tests for getCompatibleKernelRequirements are deleted.

New tests for getKernelLevel() are added.

Fixes: 139015688
Test: vintf_object_test

Change-Id: Icddc48c679e78588b436ea91fbc6ce18308386ee
5 files changed