Move array pinning out of global references table.

Move array pinning out of global references table.

We have to pin primitive arrays for certain JNI operations (it's that or
return a copy of the contents).  We don't move objects around in the
virtual heap, so simply creating a global reference is enough to ensure
the correct behavior of the calls.

The global reference implementation is changing in a way that makes
this approach inconvenient, so we now have a separate table for pinned
primitive arrays.

As a bonus, if GREF tracking is enabled, we will scan through the table
when a pin entry is added.  If there are 10 or more entries for the same
array, a complaint is logged.  This should allow us to find mismatched
Get/Release sequences much more easily (before you had to wait until the
global reference table exploded).

We currently scan pin table entries at the same time as the JNI global
references, so they'll still show up in hprof dumps as such.  (Could
also add a new Android-specific hprof root category, but that seems like
more trouble than it's worth.)