Add proguard_flags_files to r8 command line

We were not passing the proguard_flags_files files as -include
arguments to R8.

Bug: 112462307
Test: m checkbuild
Change-Id: I8f3b10fc338aa4b15c0fb220ee2891efbb0b020f
diff --git a/java/dex.go b/java/dex.go
index db240fc..8363a7d 100644
--- a/java/dex.go
+++ b/java/dex.go
@@ -111,6 +111,8 @@
 	flagFiles = append(flagFiles, j.extraProguardFlagFiles...)
 	// TODO(ccross): static android library proguard files
 
+	flagFiles = append(flagFiles, android.PathsForModuleSrc(ctx, j.deviceProperties.Optimize.Proguard_flags_files)...)
+
 	r8Flags = append(r8Flags, android.JoinWithPrefix(flagFiles.Strings(), "-include "))
 	r8Deps = append(r8Deps, flagFiles...)