Add merge_ota to otatools

Bug: 282189563
Test: manual, m otatools-package, confirm that merga_ota is
    present in otatools.zip
(cherry picked from https://android-review.googlesource.com/q/commit:6d56d45f96cc1f4e45141bfd04cd92551451d3f5)
Merged-In: I40c3866bfb016aa1dec1c6dc2d95c66deb1e61ea
Change-Id: I40c3866bfb016aa1dec1c6dc2d95c66deb1e61ea