Use shell-script $$(...) instead of $(shell ...)

Use shell-script $$(...) instead of $(shell ...)

kati doesn't support passing results of $(shell)
to other make constructs, as explained here:
https://github.com/google/kati/issues/71

Fixes the following building error in nougat-x86:

./bootable/newinstaller/Android.mk:82: kati doesn't support passing
results of $(shell) to other make constructs: date +"%F"
build/core/ninja.mk:163: recipe for target
'out/build-android_x86_64.ninja' failed
make: *** [out/build-android_x86_64.ninja] Error 1