JNI: Use PushLocalFrame/PopLocalFrame on all callbacks

JNI local references are automatically freed when returning from the
enclosing native function, but LibOpenConnect.mainloop() could
potentially run for weeks/months at a time, accumulating a large number
of dangling references.  Therefore it is necessary to garbage-collect
local objects before each callback returns to avoid overflowing the
local ref table.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>