Add root check to aflags

aflags scans the apex/ dir, which requires root access to read.
Currently the command fails with "Error: Permission denied", which
doesn't tell the user how to fix the problem. This CL adds a more
descriptive error message.

Test: adb unroot && adb shell aflags list
Bug: 347692127
Change-Id: I98a7a1ba10ef52ec47035816fa66119ea84f281d