Add `front` and `back` methods to RingBuffer

Test: Corresponding added RingBuffer tests pass
(`atest libinput_tests`)

Bug: 268245099
Change-Id: If330940e67e70d809748b2aaa6156bed835c6c9d
diff --git a/include/input/RingBuffer.h b/include/input/RingBuffer.h
index 67984b7..37fe5af 100644
--- a/include/input/RingBuffer.h
+++ b/include/input/RingBuffer.h
@@ -103,6 +103,11 @@
     iterator end() { return {*this, mSize}; }
     const_iterator end() const { return {*this, mSize}; }
 
+    reference front() { return mBuffer[mBegin]; }
+    const_reference front() const { return mBuffer[mBegin]; }
+    reference back() { return mBuffer[bufferIndex(mSize - 1)]; }
+    const_reference back() const { return mBuffer[bufferIndex(mSize - 1)]; }
+
     reference operator[](size_type i) { return mBuffer[bufferIndex(i)]; }
     const_reference operator[](size_type i) const { return mBuffer[bufferIndex(i)]; }