Add wchan data to ANR dump

Since only privileged processes with CAP_SYS_ADMIN can read kernel
stack traces from /proc/*/stack, we dump the waiting channels
instead to provide some insight as to where the process might
be stuck in the kernel.

Bug: 135458700
Fixes: 135458700
Test: adb shell am hang; Check /data/anr/<anr-file> for
      wchan data.
Change-Id: I9f13511ad89a259ce5e5465155db15d45d2c46d8