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

apex/apex.go

@@ -120,6 +120,12 @@ type apexBundleProperties struct {
 	// Default: true.
 	Installable *bool
 
+	// Whether this APEX can be compressed or not. Setting this property to false means this
+	// APEX will never be compressed. When set to true, APEX will be compressed if other
+	// conditions, e.g, target device needs to support APEX compression, are also fulfilled.
+	// Default: true.
+	Compressible *bool
+
 	// For native libraries and binaries, use the vendor variant instead of the core (platform)
 	// variant. Default is false. DO NOT use this for APEXes that are installed to the system or
 	// system_ext partition.
@@ -354,6 +360,8 @@ type apexBundle struct {
 
 	prebuiltFileToDelete string
 
+	isCompressed bool
+
 	// Path of API coverage generate file
 	coverageOutputPath android.ModuleOutPath
 }