Add "-encoding UTF-8" option to javadoc.

This fixed docs build on Mac OS X 10.9 with Java 7.
Change-Id: I91e224456c9e6c43dbf13d691c3b37d21f94b0f3
Fix droiddoc generation.

We want javadoc generated from the standard libraries
we supply and not the host standard libraries.

This also has the side effect of fixing javadoc generation
for java7 APIs that android introduced, while compiling
with java6.

bug: 8992787
Change-Id: Idebc7e12c7743a43b425ef4971f4482719fd480d
Fix javadoc error "class file for sun.util.resources.OpenListResourceBundle not found"

It seems to be a javac/javadoc bug.
See https://code.google.com/p/doclava/issues/detail?id=38

sun.util.resources.OpenListResourceBundle is a class defined in the host rt.jar,
but sometimes on some platform javadoc/javac can't find it.
-XDignore.symbol.file tells javadoc/javac to skip the stubs file ct.sym
and link against rt.jar directly.

Change-Id: I3930f7399fc14b2d6b43c29f737fa90f37515aff