Set libavb version into system property for Treble OTA

Treble allows framework-only (system.img) OTA. To libavb statically
linked in init/fs_mgr cannot parse the AVB metadata of system.img, add
this property for the Treble OTA match process.

Bug: 35236019
Test: Normal boots, use 'adb shell getprop' to check the property is set.
Test: Recovery boots, press 'Mount system' and use 'adb shell getprop'
      to check the property is set

Change-Id: I552be229c4efd45088d93252ac67176606f75d4d