Keyed on MATRIX_LEVEL_OVERRIDE, FULL_TREBLE, SHIPPING_API_LEVEL,
and SDK_VERSION.
Bug: 62136051
Test: m system_compatibility_matrix.xml -j; \
grep -e"gatekeeper" -B1 $OUT/system/compatibility_matrix.xml; \
grep -e"oemlock" -B1 $OUT/system/compatibility_matrix.xml
bullhead: gatekeeper is optional, no oemlock (legacy.xml)
marlin without COMP_LEVEL_OVERRIDE: gatekeeper is not optional,
no oemlock (26.xml)
marlin with COMP_LEVEL_OVERRIDE: gatekeeper is not optional,
oemlock exists (current.xml)
new devices with COMP_LEVEL_OVERRIDE: same as above.
Change-Id: I2ccfa9b0132bf98cc2da8f0500bea992b5fa83d0
