Reorganized rootcomponentgenerator package and added demo.

-Moved rootcomponentgenerator/annotations to dialer/inject to reduce the number of deps engineers need to include

-Move rootcomponentgenerator/processor into rootcomponentgenerator/ since the other package is now gone

-Added inject/demo and tests

TEST=unit, tap
Test: unit, tap
PiperOrigin-RevId: 191916595
Change-Id: Ic155808c7435bdce198970caa92309edb9fadac8