Limit the number of the Java source files in a single compilation unit

KYTHE_JAVA_SOURCE_BATCH_SIZE environment variable controls this setting.
The limit is 1000 if this variable is not set.

Fixes: 179932118
Test: run prebuilts/build-tools/build-prebuilts.sh, use it to build kzips, check
Change-Id: I9ad57dfd1d2c2dce5cff755b1bd61cf933420bd3