aapt2 sparse encoding bugfix

aapt2 disable sparse encoding when sdk < 32

Bug: 197642721
Test: Updated and verified affected atests pass
Change-Id: I839978207eb30f958f5d695cf8dea7f0dad1312f

aapt2 enable sparse encoding when sdk is not set.

Bug: 197642721
Test: Added new unit test and verified all tests pass
Change-Id: Ib58941d98eafe8e543d50054c9d36a51d7977c71