Enable soong build tool to handle APEX compression

1. Soong can now detect PRODUCT_COMPRESSED_APEX flag
     We don't want APEX to be compressed on all devices. Only those that
     have explicitely set PRODUCT_COMPRESSED_APEX flag.
2. Handle "compressible" field in soong build rule
     On devices that supports APEX compression, all APEX will be
     compressed by default. If any apex does not want to be compressed,
     they will need to state that by setting "compressible" field to
     false
3. Can use apex_compression_tool to compress APEX
     Note we compress the APEX after it has been signed. That way, when
     we decompress we will get a signed APEX.
4. Place the compressed APEX in system with .capex extension
     This makes it easy to identify. We still preserve the original
     extension so that when we decompress, we can just rename by cuttif
     off the .capex extension.

Note: with this change, we can create a system image with compressed
APEX, but we cannot boot with it since platform doesn't know how to
handle .capex files. Platform support will be added on follow up CLs.

Bug: 172911362
Test: OVERRIDE_PRODUCT_COMPRESSED_APEX=true m (apex_test.go)
Test: observed $OUT/system/apex has .capex files
Change-Id: I20ac4c4ceb521924c751a6017f979b2d808fdded