[MSKP] update HWUI capture script to infer package name when not given

This also changes the usage of the script, as we can't have 2 optional
values and rely on arg positioning.

Test: frameworks/base/libs/hwui/tests/scripts/skp-capture.sh -p PACKAGE_NAME -n FRAMES

Flag: TEST_ONLY

Change-Id: I470909cc0d3ae85fb977626b001938ab2bcf3604