Expose max.active.modem to be vendor inittable.

And rename it properly.

Bug: 154072245
Test: manual
Change-Id: I85b060ef90a747b43e1ab386337ffc5b49d29e57
diff --git a/private/property_contexts b/private/property_contexts
index f18a23d..2dacd88 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -489,6 +489,7 @@
 sys.usb.state           u:object_r:exported2_system_prop:s0 exact string
 
 telephony.lteOnCdmaDevice u:object_r:exported3_default_prop:s0 exact int
+telephony.active_modems.max_count u:object_r:exported3_default_prop:s0 exact int
 
 tombstoned.max_tombstone_count u:object_r:exported3_default_prop:s0 exact int