Add more context to the garbage collection log messages.

With this change, the heap size, number of bytes allocated, the
percentage of free heap, the external allocation limit, and the number
of external bytes allocated are logged.  The number of objects freed
and suspend times are no longer logged but are still included in the
event log records.

Change-Id: I5d5a9d623ae8cb72ad7d4bc5014e218c61a33561