Expose supportedAxes to frameworks/base

The list of supportedAxes are necessary for returning value of
setFontVariationSettings.

Bug: 35764323
Test: ran TextViewTest and PaintTest in cts
Change-Id: I52f244146ea0ce335df02c841f89285be2ed746e

include/minikin/FontCollection.h

@@ -53,6 +53,10 @@ public:
     std::shared_ptr<FontCollection>
             createCollectionWithVariation(const std::vector<FontVariation>& variations);
 
+    const std::unordered_set<AxisTag>& getSupportedTags() const {
+        return mSupportedAxes;
+    }
+
     uint32_t getId() const;
 
 private: