Reduce commands per shard for java protos

`m platformprotos` with a long OUT_DIR can trigger E2BIG errors in
posixspawn, reduce the commands per shard.

Test: OUT_DIR=$PWD/really-long-out-directory m platformprotos
Change-Id: Ifa4594eac7ba0e1091df0ef75e4dfc3ff366939b
diff --git a/java/proto.go b/java/proto.go
index 8731822..2fa22b6 100644
--- a/java/proto.go
+++ b/java/proto.go
@@ -24,7 +24,7 @@
 func genProto(ctx android.ModuleContext, protoFiles android.Paths, flags android.ProtoFlags) android.Paths {
 	// Shard proto files into groups of 100 to avoid having to recompile all of them if one changes and to avoid
 	// hitting command line length limits.
-	shards := android.ShardPaths(protoFiles, 100)
+	shards := android.ShardPaths(protoFiles, 50)
 
 	srcJarFiles := make(android.Paths, 0, len(shards))