Implement vendor snapshot

Vendor snapshot can be captured with "m dist vendor-snapshot". With
vendor snapshot and vndk snapshot, older version of /vendor and newer
version of /system will be able to be built together by setting
BOARD_VNDK_VERSION to past vendor's version.

Only vendor modules under AOSP are to be captured. In detail, modules
under following directories are ignored:
- device/
- vendor/
- hardware/, except for interfaces/, libhardware/, libhardware_legacy/,
  and ril/

Test modules (cc_test, etc.) and sanitized modules are also ignored.

Bug: 65377115
Test: m dist vendor-snapshot
Change-Id: If7a2f6de7f36deee936930c0ccf7c47c4a0cebf6

cc/testing.go

@@ -320,6 +320,7 @@ func CreateTestContext() *android.TestContext {
 	RegisterRequiredBuildComponentsForTest(ctx)
 	ctx.PreArchMutators(android.RegisterDefaultsPreArchMutators)
 	ctx.RegisterSingletonType("vndk-snapshot", VndkSnapshotSingleton)
+	ctx.RegisterSingletonType("vendor-snapshot", VendorSnapshotSingleton)
 
 	return ctx
 }