Different bootclasspath for droiddoc.

- If it's host module, don't set bootclasspath;
- If it's arget module,
  - It can build against the API stubs;
  - It can build against a historical SDK version;
  - It can build against core.jar

Change-Id: Id1ec3ba624bc38068b206ad7173f4facf590e021
javac: Set -source to 1.7 if using OpenJDK 7

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
Really allow the use of alternate javacc compiler

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
Increase the maximum javac heap size.

Java 7 needs more memory when you run some annotation tools.

Change-Id: Ia8b423244b9ce164500b24867fe2b3d10a5089ff