Use cp instead of install for ndk_headers

ndk_headers currently use ctx.Install to install headers in
out/soong/ndk/sysroot. The files are subsequently used to compile ndk
variants of cc libraries on host.

Since these headers are not actually installed on device, use android.Cp
to assemble the NDK sysroot. By itself, it should be a no-op, but
androd.Cp is more friendly with restricting the installation rules to
PRODUCT_PACKAGES.

To make it explicit that the sysroot is not a typical installation path,
this CL also modifies the type to OutputPath

Test: m
Bug: 332778109
Change-Id: I1131c3c764443cbaac525c6022cd09c47695d275