Revert "Icon: make Icon's Bitmaps immutable"

This reverts commit 47032a20.

Reason for revert: DroidMonitor: Potential culprit for Bug 230140012 -
verifying through ABTD before revert submission. This is part of the
standard investigation process, and does not mean your CL will be
reverted.

Change-Id: I22f5aa5bd57e37d638292773986b479428e5cdbc