Increase the Java heap size for droiddoc

Increasing the Java heap for droiddoc generation from 768 to 1280
megabytes reduces build times by about 2-3 minutes. For fast
build servers this can reduce build times by around 30%. For
slower workstation class machines the build is reduced with no
other obvious undesirable side effects.

Change-Id: I4ff04318ae78fc7b5a5a0499bacfc5527e966ff6