GC triggering performance optimizations

Copy the correct starting heap size at process init.
Interfaces to set GC/managed heap parameters.

Change-Id: If0b1637291e77f9781ae6249c5d0257f659253ca
Improve java.nio.Buffer.toString.

Bug: https://code.google.com/p/android/issues/detail?id=53637
Change-Id: I823662b39cb48b5f4456f4c58269324dfcdc2fd1