Move sharding functions for reuse

Move shardPaths and shardTests to android.ShardPaths and
android.ShardStrings for reuse in other packages.

Test: m checkbuild
Change-Id: I868802872c73616b80f56cbf11f959c01a8b793a
diff --git a/java/aapt2.go b/java/aapt2.go
index f0eb99c..cfe0dea 100644
--- a/java/aapt2.go
+++ b/java/aapt2.go
@@ -63,7 +63,7 @@
 func aapt2Compile(ctx android.ModuleContext, dir android.Path, paths android.Paths,
 	flags []string) android.WritablePaths {
 
-	shards := shardPaths(paths, AAPT2_SHARD_SIZE)
+	shards := android.ShardPaths(paths, AAPT2_SHARD_SIZE)
 
 	ret := make(android.WritablePaths, 0, len(paths))