Stop using single-hyphen metalava options

`-encoding` and `-source` have been deprecated in metalava.

`-encoding` actually does nothing apart from check to make sure that
the value is `utf-8` (in some case). Metalava always uses `utf-8`.

`-source` has been deprecated in favor of `--java-source` as metalava
also can consume kotlin and it has its own `--kotlin-source` option.

Bug: 295136054
Test: ./gradlew
(cherry picked from https://android-review.googlesource.com/q/commit:808211e519ae43517ae7d6ed5348ffb1e3b63c45)
Merged-In: I08e6931958f40022d65d417360e32b72a1d70444
Change-Id: I08e6931958f40022d65d417360e32b72a1d70444