Create system modules using .txt stubs

Create a parallel set of java_system_modules provided by the SDK. This
parallel set will build using stubs of core libraries generated from
.txt files. Since stub generation from .txt is not available at ToT yet,
these modules have been marked "enabled: false". decodeSdkDep will be
updated to use these systemModules behind a flag.

(Will create java_system_modules for core_platform in a future CL)

Test: None (will test when the enabled flag is turned on)
Change-Id: Idd89d656fcbc87e8698fe05d65a685ade4488546

java/core-libraries/Android.bp

@@ -357,3 +357,7 @@ java_system_modules {
         "art-module-intra-core-api-stubs-system-modules-lib",
     ],
 }
+
+build = [
+    "TxtStubLibraries.bp",
+]