Remove the concept of useDefaultLibs from Soong.

This field in the java/sdk structure was used in two of the many
possible configurations, so it wasn't really a "default". It also
meant that, to understand those configurations, the reader had to know
what was considered the default, which was only possibly by reading
the code in java.go and droiddoc.go which implemented special code
paths when useDefaultLibs was true. By eliminating that setting and
explicitly setting the required values, the code is simpler and easier
to understand.

This change is a straight refactoring, in the sense that the output of
the build should be unchanged.

Regarding the changes to the proguardRaiseTag dependency in java.go:
- This is a noop for anything which had sdkDep.useModule = true prior
  to this change, because they all had the same value for
  hasFrameworkLibs() and hasStandardLibs().
- This is a noop for anything which had sdkDep.useDefaultLibs = true
  prior to this change, because they do not use proguard settings.
- Therefore, it is a noop overall.
- Nevertheless, it is required to make sdkCorePlatform work. Without
  this change, such modules would pick up a dependency on framework
  libs via the (unused) proguardRaiseTag, which creates a circular
  dependency, because this is the sdk_version used when building
  framework libs themselves.

Bug: 157640067
Test: m java docs droid
Change-Id: I3a83b5edc1bd48c16b55f6f77e3e710fc8fbd8fa

java/sdk.go

@@ -412,7 +412,10 @@ func decodeSdkDep(ctx android.EarlyModuleContext, sdkContext sdkContext) sdkDep
 	switch sdkVersion.kind {
 	case sdkPrivate:
 		return sdkDep{
-			useDefaultLibs:     true,
+			useModule:          true,
+			systemModules:      config.LegacyCorePlatformSystemModules,
+			bootclasspath:      config.LegacyCorePlatformBootclasspathLibraries,
+			classpath:          config.FrameworkLibraries,
 			frameworkResModule: "framework-res",
 		}
 	case sdkNone:
@@ -434,7 +437,9 @@ func decodeSdkDep(ctx android.EarlyModuleContext, sdkContext sdkContext) sdkDep
 		}
 	case sdkCorePlatform:
 		return sdkDep{
-			useDefaultLibs:     true,
+			useModule:          true,
+			systemModules:      config.LegacyCorePlatformSystemModules,
+			bootclasspath:      config.LegacyCorePlatformBootclasspathLibraries,
 			frameworkResModule: "framework-res",
 			noFrameworksLibs:   true,
 		}