add java_api_finder plugin to mainline module DocumentsUI to generate java...

add java_api_finder plugin to mainline module DocumentsUI to generate java APIs used by DocumentsUI.

Bug: 129294170
Test: m DocumentsUI RUN_ERROR_PRONE=true |& tee documentsui_error_prone.out
Change-Id: I44f2e6657a0e036267ec6563891a2aa95f4cf0f9
Merged-In: I44f2e6657a0e036267ec6563891a2aa95f4cf0f9
(cherry picked from commit 9afb4f9a)