Fix the build breakage for droiddoc

Removed the additional "-source $javaVersion" arg in javadoc rule
otherwise javadoc will report error:
javadoc: error - The -source option may be specified no more than once.

Test: m -j ahat-docs
Bug: b/70351683
Change-Id: I7f92236884154075fb4b20cd9a363419b1ecd3f5