Initialize mSystemController before creating ProxyMediaRouter2Impl

This change removes the NPE that occurred when
ProxyMediaRouter2Impl started receiving updates from MR2ServiceImpl
before mSystemController was initialized. See the attached bug for
detailed information.

Bug: 302164392
Test: atest SystemMediaRouter2Test
Change-Id: I05459e6326949e97762419c00b72d7b254b4cef0