Refactor RouterAdvertisementDaemon to build new RA in HandlerThread

HandlerThread was introduced in aosp/2493357. This patch moves the
code originally located in buildNewRa() into the HandlerThread.

Bug: 246928127
Test: atest TetheringPrivilegedTests TetheringIntegrationTests \
      TetheringTests
Change-Id: I5f6df0a3a79d7f4a865cfec7f88bc28816b1db95