Ensure TARGET_FLATTEN_APEX == true when ro.apex.updatable is not set

TARGET_FLATTEN_APEX and ro.apex.updatable cannot be independently set.
For a device where updating APEXes is not supported, ro.apex.updatable
should not be set (or set to false) and TARGET_FLATTEN_APEX should be
set to true. For APEX-supporting devices, it is the opposite;
ro.apex.updatable == true and TARGET_FLATTEN_APEX is false (or not set).

To ensure this relationship, TARGET_FLATTEN_APEX is by default set to
true, and overridden to false when updatable_apex.mk is inherited.

Bug: 130623080
Test: choosecombo to Pixels 2 and later; get_build_var
TARGET_FLATTEN_APEX returns false.
choosecombo to the original Pixel and other non-Pixel targets;
get_build_var TARGET_FLATTEN_APEX returns true.
choosecombo to the cuttlefish; get_build_var TARGET_FLATTEN_APEX
returns false

Merged-In: Id73a594dd9838457e68e2793122592c11a84fc83
Change-Id: Id73a594dd9838457e68e2793122592c11a84fc83
(cherry picked from commit 18411a42)