Use RoSystemFeatures.hasFeatureLeanback in SystemServer

Re-route the FEATURE_LEANBACK check to use the build-time query
variant. This allows stripping of several system services when the
feature is marked unavailable.

A series of auto-generated follow-up changes will patch related
callsites more broadly.

Bug: 203143243
Test: m + presubmit
Flag: build.RELEASE_USE_SYSTEM_FEATURE_BUILD_FLAGS
Change-Id: I03fc14536a8ab19cef3079c5583c41d81f534632