Don't specify -encoding twice in Jdiff javadoc invocations.

The option '-encoding UTF-8' is included in the opts defined at L1754,
which are passed to transformJdiff (defined at L1594), and from there
to a javadoc rule (defined at L29), where they are included in a
command line string which already has '-encoding UTF-8' in it. The
result is that this appears twice in the command line. This is
apparently ignored by the version of javadoc currently used. Later
upstream versions of javadoc, however, fail with the error 'javadoc:
error - The -encoding option may be specified no more than once.'

Test: make system-api-stubs-docs
Test: make system-api-stubs-docs, using an OpenJDK 11 toolchain via OVERRIDE_ANDROID_JAVA_HOME
Bug: 131683177
Change-Id: Ib9382ada3c8259edfacb2be3ab14d4c09d8753a7