Fix bug 6558006: SystemUI native heap is huge. Fix memory leak

TextLayoutCache was leaking HB_Face objects, not freeing them when
purging the mCachedHBFaces cache. More full analysis is in the bug.

Change-Id: Ie5cd8b00c36b9d31963183c601cde49cbb73fafb

core/jni/android/graphics/TextLayoutCache.cpp

@@ -992,6 +992,10 @@ HB_Face TextLayoutShaper::getCachedHBFace(SkTypeface* typeface) {
 }
 
 void TextLayoutShaper::purgeCaches() {
+    size_t cacheSize = mCachedHBFaces.size();
+    for (size_t i = 0; i < cacheSize; i++) {
+        HB_FreeFace(mCachedHBFaces.valueAt(i));
+    }
     mCachedHBFaces.clear();
     unrefTypefaces();
     init();