set sdk_version to backstage_power_flags_lib

This change correct sdk_version for backstage_power_flags_lib. Before
the sdk_version is not set, and by default the library will be built
against the implementation of the framework. Set the sdk_version to the
right scope.

Test: presubmit
Bug: 327965477
Change-Id: Ifd2e62e42f3b42ffd19e18fc239e576b7c171dbd