Use ndk_system STL header library

Soong currently adds -isystem
prebuilts/ndk/current/sources/cxx-stl/system/include to modules that
have ndk_system STL. This does not translate well to Bazel because of
its stricter sandboxing constraints.

In preparation for building sdk variants with Bazel, create an
indirection where sdk variants that use this STL depend on a
`ndk_system` header library module.

This should be a no-op in Soong, but the resultant ninja files are not
identical. (e.g diff https://diff.googleplex.com/#key=OOLtc1GFmDDF). The
-isystem now appears _before_ the local cflags. However, this should be
fine because we have a check in `CheckBadCompilerFlags` that bans use of
-I/-isystem in user-provided cflags.

Test: diff'd ninja file
Test: TH
Bug: 298258442
Change-Id: I8c4e2b66bb9ac25c44ceedd52298ba474a554a04