Add suuport for splitting touch events across windows.

This feature is currently used to enable dragging the start and end
selection handles of a TextView at the same time.  Could be used for
other things later.

Deleted some dead code in ArrowKeyMovementMethod and CursorControllers.

Change-Id: I930accd97ca1ca1917aab8a807db2c950fc7b409

include/utils/BitSet.h

@@ -38,6 +38,9 @@ struct BitSet32 {
     // Clears the bit set.
     inline void clear() { value = 0; }
 
+    // Returns the number of marked bits in the set.
+    inline uint32_t count() const { return __builtin_popcount(value); }
+
     // Returns true if the bit set does not contain any marked bits.
     inline bool isEmpty() const { return ! value; }