ANDROID: gki_defconfig enable CONFIG_SPARSEMEM_VMEMMAP

Legacy Ion driver and SPARSEMEM for carveout regions results
in invalid page structures breaking page_to_pfn().  This can
be temporarily resolved with SPARSEMEM_VMEMMAP until the Ion
driver is refactored and can be reinvestigated.

At that time if it can be solved, or maybe correct this issue
utilizing less resources than SPARSEMEM_VMEMMAP requires. The
ABI does not change so we have the flexibility to adjust this
configuration.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 138851285
Bug: 138149732
Test: ABI_DEFINITION=common/abi_gki_aarch64.xml \
      BUILD_CONFIG=common/build.config.gki.aarch64 ./build/build_abi.sh
Change-Id: I25cc8ebe9e25260b9869c5e8d8667b280f83ca51