Allow for opt-in of proguard shrinking of tests with new flag

By adding 'shrinktests' to LOCAL_PROGUARD_ENABLED, the -dontshrink
flag will be omitted from the resulting proguard config allowing
tests to have unnecessary classes removed before dexxing.

Clockwork is running into 64k method limits on some tests which
depend on GmsCore. On 4.4W (klp-modular-dev) we don't have the
option for multi-dex so proguard is the only solution here.

This change leaves the default behavior unchanged. Shrinking is
still disabled for tests by default unless the 'shrinktests'
flag is set in the Makefile.

Change-Id: Ib586ae9e7f617072b41e95742559bf7c8a6a6ce1