[thread] add empty ThreadNetwork apex build target Add empty apex target "com.android.threadnetwork" to workaround the build issues in yaqs/5493952840252522496 FR: b/235016403 Bug: 257371610 Merged-In: If5ebeb0be7063f813b392c1c01dbf0dc834618e5 Change-Id: Ib6f1fbc585eb422d405ddf79de253e05a9be42c5