ANDROID: binder: show high watermark of alloc->pages.

Show the high watermark of the index into the alloc->pages
array, to facilitate sizing the buffer on a per-process
basis.

Change-Id: I2b40cd16628e0ee45216c51dc9b3c5b0c862032e
Signed-off-by: Martijn Coenen <maco@android.com>