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This patch enables sideloading an OTA on A/B devices while running from
recovery. Recovery accepts the same OTA package format as recent
versions of GMS, which consists of .zip file with the payload in it.

Bug: 27178350
TEST=`adb sideload` successfully a full OTA (*)
TEST=Failed to take several invalid payloads (wrong product,
fingerprint, update type, serial, etc).

<small>(*) with no postinstall script.</small>

Change-Id: I951869340100feb5a37e41fac0ee59c10095659e