Dereference cached Bitmaps in EventLogger

In Telecom, we cache the last 10 calls that have been placed in
order to provide detailed event logs during a call for bugreports.
We were also mistakenly keeping around the cached contact photo/
icon, causing unnecessarily high memory usage.

This change dereferences the Bitmaps when the calls are destroyed
so that the Bitmaps are not hanging around when the calls are
cached for Logging.

Bug: 62038439
Test: Ensure Bitmaps are not still in memory by looking at memory dump
Change-Id: Iba265a10145f2db6f57b27adee59d812ccb7ca5c