gn2bp: collect java sources

The current plan is to write a lightweight wrapper around Cronet that
serves as the Android API surface. If that holds true, we might get away
with putting all .java sources in a single jar. Otherwise, the API will
have to be put in a separate source jar.

Test: //net:net
Change-Id: I2bef893ebdf0a4390dfb634e715c60ccd94dc1fb
1 file changed