Define odm_available property to install a vendor variant to odm

'vendor_available: true' creates a vendor variant from a system
module. The vendor variant of the module is installed to /vendor.
However, we may want to install the vendor variant to /odm, instead.

'device_specific: true' does not work for this purpose because
'vendor_available: true' is allowed only for the system or product
modules to create a vendor variant. But 'device_specific: true'
itself creates a vendor variant that may not work with
'vendor_available: true'.

To install the vendor variant to /odm, we define a new property
'odm_available'. 'odm_available' is exactly the same as the
'vendor_available' except the install path of the vendor variant.
By defining 'odm_available: true', the vendor variant of the module
will be installed to /odm or /vendor/odm instead of /vendor.

Bug: 176147321
Bug: 176079978
Test: check if a module with 'odm_available: true' is installed to
      /vendor/odm
Change-Id: I2d16bd2c515796597b2fbd1eb66f7c2736434697