Don't use apexd when TARGET_FLATTEN_APEX == true

When TARGET_FLATTEN_APEX is true (= ro.apex.updatable is unset or set to
false), apexd is not used to activate the built-in flattened APEXes.
Init simply bind-mounts /system/apex to /apex.

However, there is a special case here. The runtime APEX is installed as
either /system/apex/com.android.runtime.debug or
/system/apex/com.android.runtime.release, whereas it should be activated
on /apex/com.android.runtime - without the .debug or .release suffix.
To handle that case, the build system creates an empty directory
/system/apex/com.android.runtime and the .debug or .release directory
is bind-mounted to the empty directory by init at runtime.

This change also fixes a minor bug that native watchdog is triggered
for all post-apexd processes regardless of whether ro.apex.updatable
is true or not. Since apexd is expected to shutdown itself when ro.apex
.updatable is false, we don't trigger the watchdog in that case.

Bug: 132413565
Bug: 133305381
Test: marlin and sdk_gphone are bootable
Merged-In: I219465b8b81decb960e2c5f46bab6e0768b31317
Change-Id: I219465b8b81decb960e2c5f46bab6e0768b31317
(cherry picked from commit f93088ba)