Be more defensive around invalid tzids

Add checks during boot in case the persist.sys.timezone property is set
to a bad ID.

This can happen in the rare case of a mainline rollback: i.e. if a device has
been set to a new ID and then the update is rolled back. Using GMT as a
fallback probably works without this change (it does in java.util.TimeZone),
but relies on all code, including native code that uses
persist.sys.timezone directly, knowing to interpret a bad ID as "GMT".
This commit makes that choice more explicit and defensive.

This change also removes the possibility of IOException, which is never
thrown, from some ZoneInfoDb methods.

Bug: 155738410
Test: boot with a valid id, verify persist.sys.timezone is unchanged
Test: boot with an invalid id set,  verify persist.sys.timezone is "GMT"
Merged-In: I6dc0f4f81848efbbaec6a11a62014471a0ef01fd
Change-Id: I6dc0f4f81848efbbaec6a11a62014471a0ef01fd
Exempt-From-Owner-Approval: Approved / landed internally

services/java/com/android/server/SystemServer.java

@@ -165,6 +165,8 @@ import com.android.server.wm.ActivityTaskManagerService;
 import com.android.server.wm.WindowManagerGlobalLock;
 import com.android.server.wm.WindowManagerService;
 
+import libcore.timezone.ZoneInfoDb;
+
 import dalvik.system.VMRuntime;
 
 import java.io.File;
@@ -395,8 +397,9 @@ public final class SystemServer {
             // Default the timezone property to GMT if not set.
             //
             String timezoneProperty = SystemProperties.get("persist.sys.timezone");
-            if (timezoneProperty == null || timezoneProperty.isEmpty()) {
-                Slog.w(TAG, "Timezone not set; setting to GMT.");
+            if (!isValidTimeZoneId(timezoneProperty)) {
+                Slog.w(TAG, "persist.sys.timezone is not valid (" + timezoneProperty
+                        + "); setting to GMT.");
                 SystemProperties.set("persist.sys.timezone", "GMT");
             }
 
@@ -564,6 +567,12 @@ public final class SystemServer {
         throw new RuntimeException("Main thread loop unexpectedly exited");
     }
 
+    private static boolean isValidTimeZoneId(String timezoneProperty) {
+        return timezoneProperty != null
+                && !timezoneProperty.isEmpty()
+                && ZoneInfoDb.getInstance().hasTimeZone(timezoneProperty);
+    }
+
     private boolean isFirstBootOrUpgrade() {
         return mPackageManagerService.isFirstBoot() || mPackageManagerService.isDeviceUpgrading();
     }