TARGET_FLATTEN_APEX for GSI

Regular (non-flattened) APEX may require changes to the kernel, which is
against the Treble promise. For maximum compatibility, APEXes are
flattened for GSI so that APEXes can be activated by legacy kernels.

Bug: 124159875
Test: N/A
Change-Id: Iea5701d3b917ebf8657d3ce38131c1838b51677e