GC triggering performance optimizations

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

Change-Id: If0b1637291e77f9781ae6249c5d0257f659253ca
Fix compilation of Enum on JDK 7

Change-Id: I2e58b6a527e4ef1fc7ed3359b8aea88d01a031fb

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