BACKPORT: ANDROID: Expose MGLRU admin file to sysfs

Add a sysfs interface for querying MGLRU generation counts, ages, and
sizes. Additionally kernfs_notify the admin file when a new generation
is created.

Bug: b/342318950

Change-Id: I67b8dab962b3f57e1209316a71dbc839446e4f22
Signed-off-by: Alexandre Marciano Gimenez <raging@google.com>