Apexd activates "block" apexes in vm mode

Block apexes are passed from the host via the disk image.
Each partition except the first in the disk represents an APEX.
The first partition in the disk is "signature", which contains
verification info(e.g. public key, root digest)

The signature partition should be accessed by
/dev/block/by-name/signature.

Ignore-AOSP-First: to avoid merging into S

Bug: 184605708
Bug: 181093750
Test: ApexTestCases
Merged-In: Ife39715822712df4be4de9fab27cbf7761488a5f
Change-Id: Ife39715822712df4be4de9fab27cbf7761488a5f
(cherry picked from commit 0e099c8d7f502b867ae566f66cc044bf418fdb6d)
8 files changed