init: refactor perform_apex_config

Removed --bootstrap arg. The init process knows which mount namespace is
the current one. Besides, the arg doesn't work well with the cases like
"microdroid" because there's only a single mount namespace.

This will work with the upcoming change for APEXd's mount-before-data,
which will use a single mount namespace as well.

Bug: 381173118
Flag: com.android.apex.flags.mount_before_data
Test: m & cvd create
Change-Id: I2400c314201c7b7bfe73866ae95059e398ea945a