product.prop: 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.

Copied from Coral.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Change-Id: I94f12152e778531525e7cdc6b0c9f835b9f665e4
diff --git a/product.prop b/product.prop
index 29b74bb..d703af9 100644
--- a/product.prop
+++ b/product.prop
@@ -12,23 +12,24 @@
 bluetooth.device.class_of_device=90,2,12
 
 # Set supported Bluetooth profiles to enabled
-bluetooth.profile.asha.central.enabled=true
-bluetooth.profile.a2dp.source.enabled=true
-bluetooth.profile.avrcp.target.enabled=true
-bluetooth.profile.bap.broadcast.assist.enabled=true
-bluetooth.profile.bap.unicast.client.enabled=true
-bluetooth.profile.bas.client.enabled=true
-bluetooth.profile.csip.set_coordinator.enabled=true
-bluetooth.profile.gatt.enabled=true
-bluetooth.profile.hap.client.enabled=true
-bluetooth.profile.hfp.ag.enabled=true
-bluetooth.profile.hid.device.enabled=true
-bluetooth.profile.hid.host.enabled=true
-bluetooth.profile.map.server.enabled=true
-bluetooth.profile.mcp.server.enabled=true
-bluetooth.profile.opp.enabled=true
-bluetooth.profile.pan.nap.enabled=true
-bluetooth.profile.pan.panu.enabled=true
-bluetooth.profile.pbap.server.enabled=true
-bluetooth.profile.ccp.server.enabled=true
-bluetooth.profile.vcp.controller.enabled=true
+bluetooth.profile.asha.central.enabled?=true
+bluetooth.profile.a2dp.source.enabled?=true
+bluetooth.profile.avrcp.target.enabled?=true
+bluetooth.profile.bap.broadcast.assist.enabled?=true
+bluetooth.profile.bap.unicast.client.enabled?=true
+bluetooth.profile.bas.client.enabled?=true
+bluetooth.profile.csip.set_coordinator.enabled?=true
+bluetooth.profile.gatt.enabled?=true
+bluetooth.profile.hap.client.enabled?=true
+bluetooth.profile.hfp.ag.enabled?=true
+bluetooth.profile.hid.device.enabled?=true
+bluetooth.profile.hid.host.enabled?=true
+bluetooth.profile.map.server.enabled?=true
+bluetooth.profile.mcp.server.enabled?=true
+bluetooth.profile.opp.enabled?=true
+bluetooth.profile.pan.nap.enabled?=true
+bluetooth.profile.pan.panu.enabled?=true
+bluetooth.profile.pbap.server.enabled?=true
+bluetooth.profile.sap.server.enabled?=true
+bluetooth.profile.ccp.server.enabled?=true
+bluetooth.profile.vcp.controller.enabled?=true