Update droiddoc with combined generation of Java and Kotlin docs

Due to ownership change, this CL is a combination of the two CLs:

1) 1102398: Added droiddoc module to combined dokka, doclava and add
switcher.

Added ds-docs-switched that merges outputs of dokka, doclava, and
applies switcher4 to them.

Bug: 128540915
Test: m -j ds-docs-switched

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2) 1098541: Combined target for dokka and doclava

Created a droiddoc module that creates reference docs using Kotlin.
Renamed droiddoc modules that only create one language ref-docs, and
created a genrule that merges them together.

Bug: 72394196
Bug: 138396395
Test: m -j ds-docs-kt
Test: m -j ds-docs-java
Test: m -j ds-docs

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Change-Id: If9bed782feae84e986395a9ea6401025a51bc316