Split MOUNT_FLAG_VISIBLE into MOUNT_FLAG_VISIBLE_FOR_{READ, WRITE}

Currently, VolumeInfo.isVisibleForUser(), VolumeInfo.isVisibleForRead(),
and VolumeInfo.isVisibleForWrite() are all equivalent; all of them just
mean that the volume has MOUNT_FLAG_VISIBLE.

This CL splits MOUNT_FLAG_VISIBLE into MOUNT_FLAG_VISIBLE_FOR_READ and
MOUNT_FLAG_VISIBLE_FOR_WRITE. They correspond to isVisibleForRead() and
isVisibleForWrite(), respectively, whereas isVisibleForUser() is now
defined as isVisibleForRead() || isVisibleForWrite().

Accordingly, all usages of MOUNT_FLAG_VISIBLE are replaced with
MOUNT_FLAG_VISIBLE_FOR_WRITE, and all usages of isVisibleForRead() are
replaced with isVisibleForUser().

Bug: 206019156
Test: m
Change-Id: I576430680620363f4fbe49787035bd52445b3554
Merged-In: I576430680620363f4fbe49787035bd52445b3554