Create sysprop_library soong module

A newly introduced sysprop_library soong module will generate a
java_sdk_library and a cc_library from .sysprop description files.
Both Java modules and C++ modules can link against sysprop_library
module, thus giving consistency for using generated sysprop API.

As Java controls accessibility of Internal / System properties with
@hide and @SystemApi, 2 different header files will be created. And
build system will selectively expose depending on the property owner
and the place where the client libraries go into.

Bug: 80125326
Bug: 122170616
Test: 1) Create sysprop_library module.
Test: 2) Create empty txt files under prebuilts/sdk.
Test: 3) Create api directory, make update-api, and see changes.
Test: 4) Try to link against sysprop_library with various clients.
Test: 5) Soc_specific, Device_specific, Product_specific, recovery flags
work as intended.
Change-Id: I78dc5780ccfbb4b69e5c61dec26b94e92d43c333

java/droiddoc.go

@@ -599,6 +599,9 @@ func (j *Javadoc) genSources(ctx android.ModuleContext, srcFiles android.Paths,
 		case ".aidl":
 			javaFile := genAidl(ctx, srcFile, flags.aidlFlags)
 			outSrcFiles = append(outSrcFiles, javaFile)
+		case ".sysprop":
+			javaFile := genSysprop(ctx, srcFile)
+			outSrcFiles = append(outSrcFiles, javaFile)
 		default:
 			outSrcFiles = append(outSrcFiles, srcFile)
 		}