Add visible logs about fstrim run for block based checkpoints

Bug: 120095226
Test: Tested by forcing /data/system/last-fstrim last modified time back
      2 years & manually trigger checkpoint using 'vdc checkpoint startCheckpoint 1'

Change-Id: I0cb8b6a85ae787e1ba2cdd7998a46942ca69760f
Merged-In: I0cb8b6a85ae787e1ba2cdd7998a46942ca69760f
Signed-off-by: Sandeep Patil <sspatil@google.com>