Trim reference tables when we trim the heap

Before:
System server:
 virtual                     shared   shared  private  private
  size      RSS      PSS    clean    dirty    clean    dirty    # object
2200      300      229        0       80        0      220   77 /dev/ashmem/dalvik-indirect ref table (deleted)
Location:
1896      128      102        0       28        0      100   39 /dev/ashmem/dalvik-indirect ref table (deleted)

After:
 virtual                     shared   shared  private  private
  size      RSS      PSS    clean    dirty    clean    dirty    # object
System server:
2216       64       64        0        0        0       64   79 /dev/ashmem/dalvik-indirect ref table (deleted)
Location:
2120       48       48        0        0        0       48   67 /dev/ashmem/dalvik-indirect ref table (deleted)

No pause time regression measured in memalloc test.

Bug: 17643507

Change-Id: I32d3e64cdcf8dd2f7aea509c81631597bbb9b392