Temporarily disable the new adb backup behavior

Fall back to using the legacy android:allowBackup flag for apps that
don't specify the new ALLOW_ADB_BACKUP property. This is done to not
break GTS tests that rely on adb backup since corresponding apps can't
use ALLOW_ADB_BACKUP yet (ag/13205188 is blocked with some failure that
I'm  currently investigating).

Bug: 175250895
Test: atest CtsBackupHostTestCases
Change-Id: Id40b2ae6b677afca18011077f57926518b28960f

services/backup/java/com/android/server/backup/utils/BackupEligibilityRules.java

@@ -183,7 +183,12 @@ public class BackupEligibilityRules {
                     } catch (PackageManager.NameNotFoundException e) {
                         Slog.w(TAG, "Failed to read allowAdbBackup property for + "
                                 + packageName);
-                        return false;
+
+                        // This temporarily falls back to the legacy allowBackup flag to
+                        // avoid breaking existing users of adb backup. Once they're able to use
+                        // the new ALLOW_ADB_BACKUP property, we'll return false here.
+                        // TODO(b/176088499): Return false here.
+                        return allowBackup;
                     }
                 } else {
                     // All other apps can use adb backup only when running in debuggable mode.