cryptfs: Don't use bare integers for key size

Rather than use an integer and have a comment, we use a named
constant for sizing these master key buffers.  This will help
avoid confusion when we switch to allowing different sized
master keys.

Bug: 73079191
Test: Build
Change-Id: Ifaffdd94d337bb2d5a178f818dfe00f9386ae03b
1 file changed