DSU service: Weighted progress bar

Weight progress of writable partitions less because they take much less
time to complete compared to readonly partitions. This is best effort
only and done only when we can deduce the total number of partitions
beforehand.
Also publish the weighted progress to events log so external tools can
use this value directly, instead of having to calculate this complex
progress weighting themselves.
Revise the events log field name to better reflect the "unit" the
metrics are in (size -> bytes, partition -> partition_name).
DynamicSystemInstallationService notification progress should use this
weighted progress if available.

Bug: 169036106
Bug: 224909776
Test: Start a DSU installation and observe the logcat and system notification
Change-Id: I81561c1994698387b275cab36ee058dd581f9879