Build init with the bootstrap bionic libs

As bionic libs are moved from /system to the runtime APEX, anything that
is outside of the runtime APEX cannot use private symbols from the
bionic libs.

init is not included in the runtime APEX but is using private bionic
APIs: __system_property_[add|area_init|update|...]. So, it was required
to publicize the private symbols for init.

However, since init is started before the runtime APEX is activated, it
actually cannot use the bionic libs from the runtime APEX, but will use
the bootstrap bionic libs left in /system/bootstrap/lib[64]. Because
init and the bootstrap libs are in the same partition, init doesn't need
to link against the stubs of bionic libs. In order to make this
possible, the new property 'bootstrap' is set to true allow init to use
the bootstrap bionic.

Bug: 120266448
Test: m with https://android-review.googlesource.com/c/platform/bionic/+/849044

Change-Id: If61f25faf9aed4968bf2922859ceb94276ba03fb