Add updatable_apex.mk

The product abstracts configs that need to be set to support updating of
APEXes.

Bug: 130418467
Test: m
Test: adb shell getprop ro.apex.updatable in Pixel 2 and 3
Test: adb shell /system/apex/com.android.apex.cts.shim.apex exists.
Merged-In: Idb44388a0cf6338f8b4500a1d0d2762ed59476bf
Change-Id: Idb44388a0cf6338f8b4500a1d0d2762ed59476bf
(cherry picked from commit fa51bedf)