ANDROID: dm-bow: Fix not to skip trim at framented range

If free blocks hole is smaller than discard_granularity,
TRIM to this range can be skipped.
Fix this by changing the granularity to 4kb at dm-bow layer,
not to skip TRIM to every tiny free blocks.

Bug: 154411183
Signed-off-by: hyeongseok.kim <hyeongseok@gmail.com>
Cc: hyeongseok.kim <hyeongseok.kim@lge.com>
Change-Id: Ic7c33d94a016d0ad5a75514eae1056c328c9c1ba