Add missing final and @GuardedBy

This is a follow up CL to my previous CL [1], where I forgot to add
final and @GuardedBy to InputMethodManagerService#mStartInputMap.

 [1]: I9921b381e02106dbffff5e0b3d13f0a1245ce807
      6db3bfe3

Test: Compile
Bug: 35079353
Change-Id: I29897752e16573a27bf700b1d822889789c35292