[nearby] Add nearby_enable_ble_in_init flag

Bug: 312430336
Ignore-AOSP-First: nearby_not_in_aosp_yet
Test: -m
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:fc084ea87ce18275c788c21ba9018636dfdff5e0)
Merged-In: I5bafa8a85d4f2169fcdef73e2d9f21534553ff72
Change-Id: I5bafa8a85d4f2169fcdef73e2d9f21534553ff72