Track stack memory in MemoryInfo.

In /proc/pid/maps the main stack is identified as "[stack]". In Linux 3.4
thread stacks are identified as "[stack:<tid>]" where <tid> is the thread ID of
the thread using that stack. Add support in MemInfo's otherStats to track
memory being used by the stack.

Add sanity check on the length of the otherStats array.
Add detail messages to two null pointer exceptions.

Bug: 8265251

Change-Id: I2b228b147da9bdc5c7a4d825dfc628fcc7923927