Initial system feature codegen prototype

Add a simple codegen utility for system features that:

  * Exports a `hasFeatureX` method for each defined system feature
  * Hardcodes the result if readonly system features are enabled
  * Falls back to a runtime query otherwise

Note that this tool is not yet used, referenced, or enabled in any
production code. Follow-up changes will route build flags into
inputs for the codegen, and the outputs will be wired into the
framework.

Test: atest systemfeatures-gen-tests --host
Bug: 203143243
Change-Id: I059a38b11305c759662ea6c74c11d3cfdb058c40