Add --unsigned_payload_only flag to apexer

The flag instructs apexer to output the payload before it is signed by
avbtool.

The CL also contains new test to ensure that when the unsigned payload
is signed by avbtool, it is same as the payload we get when we unzip an
apex.

Bug: 149993331
Test: atest --host apexer_test
Change-Id: I0c0c29ee2fabaa447f5844e71949a00778b1d935
3 files changed