bp2build: Split export_{includes,system_includes}

The specification of exporting includes vs system includes has an impact
on inclusion sort order. Conflating the two caused some symbols to not
be resolved correctly.

Bug: 198403271
Test: build/bazel/ci/bp2build.sh
Test: USE_BAZEL_ANALYSIS=1 m libbacktrace_no_dex succeeds with libc++_*
      modules removed from mixed build denylist (would fail otherwise)
Change-Id: I08aff253d8962dc678ed10214b1c171330e0fe19

bp2build/cc_library_conversion_test.go

@@ -122,8 +122,8 @@ cc_library {
         "-I.",
         "-I$(BINDIR)/.",
     ],
+    export_includes = ["foo-dir"],
     implementation_deps = [":some-headers"],
-    includes = ["foo-dir"],
     linkopts = ["-Wl,--exclude-libs=bar.a"] + select({
         "//build/bazel/platforms/arch:x86": ["-Wl,--exclude-libs=baz.a"],
         "//build/bazel/platforms/arch:x86_64": ["-Wl,--exclude-libs=qux.a"],