build: Rename Plat->SystemExt*SepolicyDirs

Depends on changes in build/soong and build/make.

Make it more consistent with the change of `BOARD_PLAT_*`
behaviour and prepares introduction of
`SYSTEM_EXT_{PUBLIC,PRIVATE}_SEPOLICY_DIRS`.

Test: `make selinux_policy`with `BOARD_PLAT_PUBLIC_SEPOLICY_DIR` and
     `BOARD_PLAT_PRIVATE_SEPOLICY_DIR` set.

Signed-off-by: Felix <google@ix5.org>
Change-Id: I9885f0594da9b76ee51386a980dfa76af98e2171
1 file changed