init_gsi: implement phh's qemu.hw.keys persistent prop

init/init_gsi.rc

@@ -3,3 +3,6 @@ on late-init
 
 on post-fs
     exec u:r:init_gsi:s0 root -- /system_ext/bin/init_gsi
+
+on property:persist.sys.gsi.hw.mainkeys=*
+    setprop qemu.hw.mainkeys ${persist.sys.gsi.hw.mainkeys}

system.prop

@@ -10,3 +10,5 @@ persist.dbg.volte_avail_ovr=1
 persist.dbg.vt_avail_ovr=1
 persist.dbg.wfc_avail_ovr=1
 persist.dbg.allow_ims_off=1
+# Enable hw keys by default (unless overridden by user)
+persist.sys.gsi.hw.mainkeys=0