Make SUPPORT_MULTI_SIM recognizable in bp

This commit establishes a soong config variable `sim_count` for
easier converting Android.mk to Android.bp when dealing with multiple
SIMs.

Bug: 343823190
Test: presubmit
Change-Id: I8226003df3d9f669df6e2af12310ad73e5786a74
diff --git a/device.mk b/device.mk
index 3022a34..c50291b 100644
--- a/device.mk
+++ b/device.mk
@@ -904,6 +904,7 @@
 # Exynos RIL and telephony
 # Multi SIM(DSDS)
 SIM_COUNT := 2
+$(call soong_config_set,sim,sim_count,$(SIM_COUNT))
 SUPPORT_MULTI_SIM := true
 # Support NR
 SUPPORT_NR := true