Fork Pager from Accompanist

This CL forks the Pager API from Accompanist. Note that I took the
version 0.20.0 of Accompanist [1], which is the version right before the
Pager API starts depending on dev.chrisbanes.snapper [1], which I'm not
sure is ok to fork in the platform.

[1] https://github.com/google/accompanist/releases/tag/v0.20.0
[2] https://github.com/google/accompanist/commit/498301e4d416b831e97e34335242fe7055e440df

Bug: 247473910
Test: Manual
Change-Id: I57947a196be84af9c34365377588e50b8f249af0