[java9] Add "--dex-api" option to Metalava.

This allow Metalava to generate public-dex file when user specify
dex_api_filename.

This blocks how we try java9 for droiddoc since "hiddenapi-lists-docs"
targets depends on libcore sources.

Test: m -j hiddenapi-lists-docs
Bug: b/78245848

Change-Id: Ic847c1123b14afb599dbf2a588885fd36e56b2cc
Merged-In: Ic847c1123b14afb599dbf2a588885fd36e56b2cc