Silence trailing "1 warning" message from javac when warning is silenced

soong_javac_wrapper is silencing a useless warning:
warning: [options] bootstrap class path not set in conjunction with -source 1.9
but recent versions of javac have started also printing:
1 warning

Read the warning count, subtract the number of silenced warnings, and
reprint it if the non-silenced warning count is nonzero.

Fixes: 144118634
Test: javac_wrapper_test.go
Change-Id: Ie1d0a978188ab7b1c41027f718a1274608628123