Add --no-ndk to ndkstubgen

By default, ndkstubgen does not omit NDK symbols as long as they
satisfy the API version and the CPU architecture requirements. This
change adds a new flag --no-ndk to ndkstubgen which is used to
override the default behavior. If the flag is set, NDK symbols are
omitted leaving only the annotated symbols (e.g. #apex, #systemapi,
etc.).

This will be used for stub libraries that are not part of NDK. So far,
symbols in such libraries haven't needed to be annotated as #apex, and
that has caused a confusion that those symbols belong to NDK. The
follow-up change will ensure that those symbols are always annoated as
either #apex or #systemapi so that their roles are clearly visible.

Bug: 184712170
Test: atest test_ndkstubgen
Test: atest test_symbolfile
Change-Id: Ic8d2c7d0b32bdef79f7563621035e60f406e4131