Explicitly tag max-target-o APIs as "lo-prio".

This allows the presubmit checker to explicitly allow certain changes to
these APIs, rather than special casing tha max-target-o list. This in turn
will allow other APIs to be tagged as lo-prio in future to get the same
treatment.

Test: m out/soong/hiddenapi/hiddenapi-flags.csv
Bug: 172993934
Change-Id: Ic7cc0243b2b018c96d420a3f266c4b3a6958c32f
diff --git a/java/hiddenapi_singleton.go b/java/hiddenapi_singleton.go
index c3ae75e..4003aea 100644
--- a/java/hiddenapi_singleton.go
+++ b/java/hiddenapi_singleton.go
@@ -264,7 +264,7 @@
 		FlagWithInput("--max-target-p ",
 			android.PathForSource(ctx, "frameworks/base/config/hiddenapi-max-target-p.txt")).
 		FlagWithInput("--max-target-o ", android.PathForSource(
-			ctx, "frameworks/base/config/hiddenapi-max-target-o.txt")).Flag("--ignore-conflicts ").
+			ctx, "frameworks/base/config/hiddenapi-max-target-o.txt")).Flag("--ignore-conflicts ").FlagWithArg("--tag ", "lo-prio").
 		FlagWithInput("--blocked ",
 			android.PathForSource(ctx, "frameworks/base/config/hiddenapi-force-blocked.txt")).
 		FlagWithInput("--unsupported ", android.PathForSource(