Fix issue with misaligned glowpad handle

If the drawable for the handle changes, a new TargetDrawable is
created (without the correct positional parameters). This causes it
to be misaligned (e.g. during call waiting).

To fix this, simply replace the drawable without creating the TargetDrawable
from scratch. This preserves its positionX and positionY variables.

Bug: 20860087
Change-Id: I696bddf2d6d92cd8f17a3818367e9c0ac71b4a63