Add soong_droiddoc_prebuilt.mk

Soong droiddoc modules were using soong_java_prebuilt.mk, but they
don't need any of the java logic from it, and it includes
base_rules.mk which was not included by droiddoc.mk.  Add
soong_droiddoc_prebuilt.mk that just installs the droiddoc
outputs without including base_rules.mk.

Fixes building docs modules in mm, mma, and m checkbuild.

Bug: 112388925
Test: m docs
Change-Id: I4f7262e2c3b1afbee44c1cc306330885b687a074