Fix compilation of Enum on JDK 7

 This is the 2nd commit message:

libcore: Better Enum fix for JDK 7

 * Jerome Pilliet suggests using a cast in the compareTo method instead of
   changing the field visibility. This is a much better fix.

Change-Id: I334105e0922225512a4d0a2d4b6c9c6ec71b9c51
GC triggering performance optimizations

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

Change-Id: If0b1637291e77f9781ae6249c5d0257f659253ca