ANDROID: sdcardfs: Add option to drop unused dentries

This adds the nocache mount option, which will cause sdcardfs to always
drop dentries that are not in use, preventing cached entries from
holding on to lower dentries, which could  cause strange behavior when
bypassing the sdcardfs layer and directly changing the lower fs.

Change-Id: I70268584a20b989ae8cfdd278a2e4fa1605217fb
Signed-off-by: Daniel Rosenberg <drosen@google.com>