Add DSU default userdata partition size

Use DEFAULT_USERDATA_SIZE as userdata partition size if the calling
Intent desn't specify the userdata size or the specified size is zero.

Bug: 145891687
Test: adb shell am start-activity \
  -n com.android.dynsystem/com.android.dynsystem.VerificationActivity \
  -a android.os.image.action.START_INSTALL \
  -d file:///storage/emulated/0/Download/aosp_arm64-dsu_test.zip

Change-Id: I52767dfefef394403c5e16fcfd40792b14ff9a71