Fix build with modern NDK and policy parsing on newer Android devices

[datalocaltmp]

NDK build fixes:

• Application.mk: drop deprecated ABIs (armeabi, mips, mips64) and bump APP_PLATFORM to android-21, required by NDK r23+
• libapol/util.c: guard swab() definition with #ifndef BIONIC since newer Bionic already defines it as an inline in <unistd.h>
• libapol/render.c: add #include <unistd.h> so the Bionic inline swab() is visible at compile time (fixes linker error in render.c)

Policy parsing fixes:

• libsepol/avtab.h: add AVTAB_XPERMS_NLMSG (0x03) for netlink message extended permissions added in newer kernels
• libsepol/avtab.c: add AVTAB_NEVERALLOW and AVTAB_XPERMS_NEVERALLOW to spec_order[] so runtime-neverallow policy entries are accepted; extend XPERMS read branch to cover AVTAB_XPERMS_NEVERALLOW; add AVTAB_XPERMS_NLMSG to the Android-M compat heuristic exclusion list so policies containing nlmsg xperms entries are not mistakenly parsed in compat mode (which consumed one fewer byte per entry, desyncing the entire avtab stream)