Remove android.test.base from bootclasspath

Default to exclude android.test.base from the bootclasspath unless the
REMOVE_ATB_FROM_BCP flag is set to false.

Bug: 73711752
Test: make droid, flash, atest -p cts/tests/signature/
Change-Id: Iabcda707ff0f0b4ec7cade1125938a2759af8f76