stagefright: misc preliminary fixes for HEIF support

- Support 32bit RGB in ColorConverter.

- Allow dst crop to be not the entire dst frame buffer.

- Also has a minor fix in CallbackDatasource to use left over
cache first before reading from source. This helps to reduce
backward reads to the source.

bug: 64077740

Test: cts-tradefed run cts-dev --module CtsMediaTestCases --compatibility:module-arg CtsMediaTestCases:include-annotation:android.platform.test.annotations.RequiresDevice

Change-Id: Ia7bdb9a48b27a312af7cbe4a97f14382a8cb6278