Skip MIPS headers when generating NDK prebuilts

When generating NDK prebuilts, skip installing MIPS headers, but keep
them when doing regular platform build. Ndk_abis property is only set
to true with build/soong/scripts/build-ndk-prebuilts.sh

Revert this once MIPS is supported in NDK again.

This effectively skips installing the following header modules when
creating NDK prebuilts:
- libc_asm_mips
- libc_asm_mips64

This fixes the issue introduced by commit:
050f046 Remove mips/mips64 headers from the NDK sysroot.

Test: build/soong/scripts/build-ndk-prebuilts.sh
Test: build aosp_mips[64]-eng

Change-Id: I3af73649bc8ac304bca0e21ec81f156abe546c90
Signed-off-by: Lazar Trsic <Lazar.Trsic@mips.com>