Capture list of unused methods when shrinking in R8

Use the -printusage flag in R8 to output a list of the unused
methods.  Some of the files can be large (2MB for DocumentsUI,
87MB for all of AOSP), so immediately zip them and remove the
originals.  The zipped files will be merged and disted.

Bug: 151857441
Test: m TARGET_BUILD_APPS=DocumentsUI dist
Change-Id: I780e84e80eba7fe4d4fa15fec0f461890afd900b