updatable_apex.mk is not inherited from mainline_system.mk

mainline_system.mk is inherited by not only Pixels but also products
like GSI and sdk_gphone where kernel isn't capable of supporting
updatable APEX.

Stop inheriting updatable_apex.mk from mainline_system.mk and let each
product to decide whether or not to inherit updatable_apex.mk.

Also, TARGET_FLATTEN_APEX is not set to true for GSI as it is the
default setting.

Bug: 132664085
Test: choosecombo 1 aosp_arm64 userdebug; get_build_var
TARGET_FLATTEN_APEX shows true
choosecombo 1 sdk_gphone userdebug; get_build_var TARGET_FLATTEN_APEX
shows true

Change-Id: I3c4e1e4b2bbebd675904d148dd18acb57ecbdfe9