Use --sysroot when compiling against the NDK

Match the NDK build by using --sysroot to point to the NDK headers
rather than manually specifying the include directories with -isystem.
Also remove --nostdlibinc from the global cflags so it is not used
for NDK builds, and manually add it back for non-NDK builds.

Bug: 374130155
Test: builds
Flag: EXEMPT refactor
Change-Id: I20d5605a3154bf51d64132ac1d8ea427eda32066