Support copy-on-write persistent data block when running a DSU

The persistent data block is protected when running a DSU. This CL
adds a copy-on-write scratchpad for the persistent data block and let
the test environment to have an emulated writable pdb service.

Bug: 175852148
Test: gsi_tool install & pass the vts_kernel_net_tests
Test: gts-tradefed run gts -m GtsOemLockServiceTestCases -t com.google.android.oemlock.gts.OemLockServiceTest
Test: cts-tradefed run cts -m CtsPermission2TestCases -t android.permission2.cts.PrivappPermissionsTest

Change-Id: I8af15775dbc5f8c570bd4eb4faec5036b7b43ebc