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: I93d355e30771868f1c4664eebbce579e1e37cdcb

device.mk

@@ -931,7 +931,9 @@ PRODUCT_PACKAGES += \
 # 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
 # Support 5G on both stacks