Print number of converted modules

Previously this was equal to the number of generated targets, but with
recent cc_library changes, we now sometimes generate more than one BUILD
target for a single module. Thus, converted module count is a more
useful metric.

At time of writing, this outputs:
`Converted 390 Android.bp modules to 453 total generated BUILD targets.
Included 4 handcrafted BUILD targets. There are 45093 total Android.bp
modules.`

Test: m bp2build
Change-Id: I7d68880dd4c6bf649ca753837a7f3a9c0d73753a