Soong: support Errorprone javacflags

Add support for an errorprone block in Java modules that accepts
additional javacflags for the errorprone build.

Sample:

    errorprone: {
        javacflags: ["-Xep:EqualsNaN:WARN"],
    },

Bug: 72004718
Test: m RUN_ERROR_PRONE=true
Test: manual - add block with ERROR to a project
Test: androidmk_test
Change-Id: I502248fe76c26aa19102f413af72a7324c35b7f4