[nearby] Add disable logic

Also moves enableBLE logic from init to when there is at least one
client

Test: -m
Bug: 294906731
Ignore-AOSP-First: nearby_not_in_aosp_yet
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:3f5aea69f9f36097b407822b0cc177c0df51f061)
Merged-In: I24ca7581d60afee484d14f66a411344ce132425a
Change-Id: I24ca7581d60afee484d14f66a411344ce132425a