Introduce a system feature annotation processor

Add a system feature annotation processor that can be used to compute
various useful properties for system features, putting them in a single
SystemFeaturesMetadata class. To start, generate the number of system
features in PackageManager as annotated with
@SdkConstant(SdkConstantType.FEATURE).

A follow-up change will add this processor as a plugin to the framework,
using the feature count as a size hint for the system feature IPC cache.

A separate follow-up will augment the codegen such that we can compute a
"perfect" indexing of SDK system features to indices. This can then be
used to optimize system feature queries without paying a (relatively)
high per-process memory cost for a runtime, string-based hash.

Bug: 203143243
Test: atest systemfeatures-gen-tests
Change-Id: I70f0f148cd644e3f38648f023df13624004766f0