Add dist-for-goals for droiddoc docs.zip

Old droiddoc will install the *-docs.zip & API files to the dist.

And also remove INTERNAL_PLATFORM_*API_FILE variables from config.mk and
get rid of the dependencies on api-stubs, system-api-stubs, test-api-stubs
timestamp.

Test: m -j api-stubs-docs
Bug: b/70351683
Change-Id: I08923bb5f62d2f55961d9a2865723d7b3fae7206
Merged-In: I08923bb5f62d2f55961d9a2865723d7b3fae7206