Use utf-8 instead of default encoding in build_android_stubs.mk

When not explicitly setting the encoding to use for 'javac' it uses some
default encoding - this will fail when using ascii as default encoding
and compiling files containing utf-8 characters.

Bug: 34098662
Test: ran 'make' locally (internal master) for an Angler build.
Change-Id: I9dbdabbfa69f04c52e09285d85494e0f0d188518