Symbols that have been added to a library but should not be exposed
in any of the current NDK API levels should be tagged with "future".
These will be suppressed from the NDK libraries.
Once all this is in better shape we'll have `sdk: "current"`. Symbols
tagged with "future" will be available for that.
Note that this tag can be applied directly to a version. Aside from
being more ergonomic than tagging an entire section, this also solved
the problem of gen_stub_libs.py emitting an empty global section
(which is not valid syntax) in the case where every symbol is
"future". Tag the version instead and it will be omitted.
Test: `make ndk` with libc/libm migration patches.
Change-Id: I41f6e4939c406f695ab5725f360ec6554ad8ab31