Add feature versions for devices and apps.

We're starting to see more instances of device features that will
increment separately from the SDK API level, such as camera HAL,
GPU capabilities, Bluetooth, and other hardware standards.

This change adds the ability for device features to specify a
version, which is defined to be backwards compatible.  That is, apps
requesting an older version of a feature must continue working on
devices with a newer version of that same feature.

When a version is undefined, we assume the default version "0".

Bug: 27162500
Change-Id: If890bf3f3dbb715e8feb80e7059a0d65618482ea