Dedup registration code for module types and singletons

The registration of module types and singletons is duplicated between
init() functions that register them for use in the build runtime and
test context creation code that registers them for testing.

This is a proof of concept for a mechanism that will allow the code
to be shared. It defines a RegistrationContext interface that is
implemented by both the TestContext and the new initRegistrationContext
type. An instance of the the latter is available through the
InitRegistrationContext variable.

The intent is that the registration of the module types and singleton
types will be extracted from the init() function into a separate
function that takes a RegistrationContext parameter. That method is
called from init() passing in the InitRegistrationContext and from a
test passing in the TestContext. Something like this:

  func init() {
    RegisterBuildComponents(android.InitRegistrationContext)
  }

  func RegisterBuildComponents(ctx android.RegistrationContext) {
    ctx.RegisterModuleType(....)
    ....
  }

A test would do something like this:

  ctx := android.NewTestContext()
  RegisterBuildComponents(ctx)

Test: m nothing
Change-Id: I97173cabb6d6cf7ce98fdb5f73418438b1997b35