FP3: Use the ?= operator so BT properties can be overridden

System properties set with the "=" operator cannot be overriden. This
device is used elsewhere in the system and some of the properties need
to be overridden. The "?=" operator will use the assigned value if no
other values are assigned elsewhere, allowing for overrides.

Bug: 230638462
Test: adb shell getprop | grep bluetooth
Change-Id: I12dfdf4830e041b2c4e50eb8136ac0fb7146cc8e
(cherry picked from commit 92f2c0e6a3fcbd63ad1805e78e3bc50c46d2ba45)