Check sysprop value before trying to bind to service

TbsService and HeadsetService were missing one check
before BluetoothManagerService would try to bond to them.

Bug: 236196227
Tag: #feature
Test: manual
Ignore-AOSP-First: will be cherry-picked
Change-Id: Iceb6c437216092a7dccc238f396f4d95e341af1d