For fuse read ahead can be configured by writing a value to the
/sys/class/bdi/{MAJOR}:{MINOR}/read_ahead_kb file.
There are several different ways of getting {MAJOR}:{MINOR} values of
the filesystem:
* Look at st_dev of stat("/mnt/user/0/emulated").
* Parse /proc/self/mountinfo.
Stat'ing approach is used since it's easier to implement.
Bug: 157982297
Test: atest vold_tests
Test: adb shell cat /proc/self/mountinfo to get MAJOR:MINOR
Test: adb shell cat /sys/class/bdi/{MAJOR}:{MINOR}/read_ahead_kb
Test: created public volume, checked it's read_ahead_kb is also 256
Change-Id: Id0c149c4af1ceabf3afc33b4100563a512b38316
dcee5c1d21